In both cases, its abbreviation is what is actually displayed.Let H N be the amount of header overhead required by N files.Because N has to be an integer, and you can only fit so many files before the kernel size drops to zero, it suffices to test every possible value of N and select the one that yields the most output.It can be used to group elements for styling purposes using the class or id attributesor because they share attribute values, such as lang.The construction depends on features of both zip and DEFLATE—it is not directly portable to other file formats or compression algorithms.People comment how they love a personalized video.This algorithm is intended to mitigate security dangers involved in downloading files from untrusted sites, and user agents are strongly urged to follow it.Their value is a URLserialized.You could reverse the order of the entries in the central directory, for example, and see whether the zip files are still detected.This keyword does not create a hyperlinkbut annotates any other hyperlinks created by the element the implied hyperlink, if no other keywords create one.A zip bomb is usually a small file for ease of transport and to avoid suspicion.Content model : Transparent.The prev keyword indicates that the document is part of a sequence, and that the link is leading to the document that is the previous logical document in the sequence.There are two kinds of links in HTML:.Build better business relationships with video messaging.
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A better zip bomb – Sales Customer Success Internal Communication.The any keyword represents that the resource contains a scalable icon, e.A zip bomb allows a program to function normally, but, instead of hijacking the program’s operation, creates an archive that requires an excessive amount of time, disk space, or memory to unpack.The source code has a –template option to facilitate this use case.The manifest keyword may be used with link elements.If the element is not a child of a dl element: flow content.
Zip is a container format, not a compression algorithm.This description of the zip format omits many details that are not needed for understanding the zip bomb.For full information, refer to section 4.By compressing a long string of repeated bytes, we can produce a kernel of highly compressed data.By itself, the kernel’s compression ratio cannot exceed the DEFLATE limit of , so we want a way to reuse the kernel in many files, without making a separate copy of it in each file.
We can do it by overlapping files: making many central directory headers point to a single file, whose data is the kernel.Let’s look at an example to see how this construction affects the compression ratio.
Suppose the kernel is bytes and decompresses to 1 MB.Then the first MB of output “costs” bytes of input:.But every 1 MB of output after the first costs only 47 bytes — we don’t need another local file header or another copy of the kernel, only an additional central directory header.
A bigger kernel raises the ceiling.The problem with this idea is a lack of compatibility.Because many central directory headers point to a single local file header, the metadata—specifically the filename—cannot match for every file.Some parsers balk at that.And the Python zipfile module throws an exception :.
Next we will see how to modify the construction for consistency of filenames, while still retaining most of the advantage of overlapping files.
We need to separate the local file headers for each file, while still reusing a single kernel.Simply concatenating all the local file headers does not work, because the zip parser will find a local file header where it expects to find the beginning of a DEFLATE stream.But the idea will work, with a minor modification.Every local file header except the first will be interpreted in two ways: as code part of the structure of the zip file and as data part of the contents of a file.
Compressed blocks are what we usually think of; for example the kernel is one big compressed block.But there are also non-compressed blocks, which start with a 5-byte header with a length field that means simply, “output the next n bytes verbatim.
The output is the concatenation of decompressing all the blocks in order.The “non-compressed” notion only has meaning at the DEFLATE layer; the file data still counts as “compressed” at the zip layer, no matter what kind of blocks are used.It is easiest to understand this quoted-overlap construction from the inside out, beginning with the last file and working backwards to the first.Start by inserting the kernel, which will form the end of file data for every file.
Now prepend a 5-byte non-compressed block header colored green in the diagram whose length field is equal to the size of LFH N.Set the “compressed size” metadata field in both of the new headers to the compressed size of the kernel plus the size of the non-compressed block header 5 bytes plus the size of LFH N.At this point the zip file contains two files, named “Y” and “Z”.
Let’s walk through what a zip parser would see while parsing it.Suppose the compressed size of the kernel is bytes and the size of LFH N is 31 bytes.The first file’s filename is “Y” and the compressed size of its file data is bytes.Interpreting the next bytes as a DEFLATE stream, we first encounter the 5-byte header of a non-compressed block that says to copy the next 31 bytes.
Now we have reached the end of the compressed data and are done with file “Y”.Now we have reached the end of the final file and are done.The output file “Z” contains the decompressed kernel; the output file “Y” is the same, but additionally prefixed by the 31 bytes of LFH N.We complete the construction by repeating the quoting procedure until the zip file contains the desired number of files.
Each new file adds a central directory header, a local file header, and a non-compressed block to quote the immediately succeeding local file header.Compressed file data is generally a chain of DEFLATE non-compressed blocks the quoted local file headers followed by the compressed kernel.
The output files are not all the same size: those that appear earlier in the zip file are larger than those that appear later, because they contain more quoted local file headers.
The contents of the output files are not particularly meaningful, but no one said they had to make sense.This quoted-overlap construction has better compatibility than the full-overlap construction of the previous section, but the compatibility comes at the expense of the compression ratio.There, each added file cost only a central directory header; here, it costs a central directory header, a local file header, and another 5 bytes for the quoting header.
Now that we have the basic zip bomb construction, we will try to make it as efficient as possible.We want to answer two questions:.It pays to compress the kernel as densely as possible, because every decompressed byte gets magnified by a factor of N.All decent DEFLATE compressors will approach a compression ratio of when given an infinite stream of repeating bytes, but we care more about specific finite sizes than asymptotics.For our purposes, filenames are mostly dead weight.
While filenames do contribute something to the output size by virtue of being part of quoted local file headers, a byte in a filename does not contribute nearly as much as a byte in the kernel.We want filenames to be as short as possible, while keeping them all distinct, and subject to compatibility considerations.The first compatibility consideration is character encoding.
TXT Appendix D.But this is a major point of incompatibility across zip parsers, which may interpret filenames as being in some fixed or locale-specific encoding.We are further restricted by filesystem naming limitations.Some filesystems are case-insensitive, so “a” and “A” do not count as distinct names.As a safe but not necessarily optimal compromise, our zip bomb will use filenames consisting of characters drawn from a character alphabet that does not rely on case distinctions or use special characters:.
Filenames are generated in the obvious way, cycling each position through the possible characters and adding a position on overflow:.There are 36 filenames of length 1, 36 2 filenames of length 2, and so on.Given that the N filenames in the zip file are generally not all of the same length, which way should we order them, shortest to longest or longest to shortest? A little reflection shows that it is better to put the longest names last, because those names are the most quoted.
Ordering filenames longest last adds over MB of output to zblg.It is a minor optimization, though, as those MB comprise only 0.The quoted-overlap construction allows us to place a compressed kernel of data, and then cheaply copy it many times.For a given zip file size X , how much space should we devote to storing the kernel, and how much to making copies? To find the optimum balance, we only have to optimize the single variable N , the number of files in the zip file.
Every value of N requires a certain amount of overhead for central directory headers, local file headers, quoting block headers, and filenames.All the remaining space can be taken up by the kernel.Because N has to be an integer, and you can only fit so many files before the kernel size drops to zero, it suffices to test every possible value of N and select the one that yields the most output.It is not a coincidence.Let’s look at a simplified model of the quoted-overlap construction.In the simplified model, we ignore filenames, as well as the slight increase in output file size due to quoting local file headers.
Analysis of the simplified model will show that the optimum split between kernel and file headers is approximately even, and that the output size grows quadratically when allocation is optimal.Let H N be the amount of header overhead required by N files.Refer to the diagram to understand where this formula comes from.In this simplified model we ignore the minor additional expansion from quoted local file headers.
Taking the derivative and finding the zero gives us N OPT , the optimal number of files.H N OPT gives the optimal amount of space to allocate for file headers.From this we see that the output size grows quadratically in the input size.As we make the zip file larger, eventually we run into the limits of the zip format.It happens that the first limit we hit is the one on uncompressed file size.Accepting that we cannot increase N nor the size of the kernel without bound, we would like find the maximum compression ratio achievable while remaining within the limits of the zip format.
The way to proceed is to make the kernel as large as possible, and have the maximum number of files.Even though we can no longer maintain the roughly even split between kernel and file headers, each added file does increase the compression ratio—just not as fast as it would if we were able to keep growing the kernel, too.
In fact, as we add files we will need to decrease the size of the kernel to make room for the maximum file size that gets slightly larger with each added file.Any major improvements to the compression ratio can only come from reducing the input size, not increasing the output size.Among the metadata in the central directory header and local file header is a CRC checksum of the uncompressed file data.
This poses a problem, because directly calculating the CRC of each file requires doing work proportional to the total unzipped size, which is large by design.It’s a zip bomb, after all.We would prefer to do work that in the worst case is proportional to the zipped size.Two factors work in our advantage: all files share a common suffix the kernel , and the uncompressed kernel is a string of repeated bytes.
We will represent CRC as a matrix product—this will allow us not only to compute the checksum of the kernel quickly, but also to reuse computation across files.
You can model CRC as a state machine that updates a bit state register for each incoming bit.The basic update operations for a 0 bit and a 1 bit are:.To see why, observe that multiplying a matrix by a vector is just summing the columns of the matrix, after multiplying each column by the corresponding element of the vector.
This representation is called homogeneous coordinates.The matrices M 0 and M 1 are shown.The benefit of a matrix representation is that matrices compose.Suppose we want to represent the state change effected by processing the ASCII character ‘a’, whose binary representation is 2.We can represent the cumulative CRC state change of those 8 bits in a single transformation matrix:.
And we can represent the state change of a string of repeated ‘a’s by multiplying many copies of M a together—matrix exponentiation.
For example, the matrix representing the state change of a string of 9 ‘a’s is.The square-and-multiply algorithm is useful for computing M kernel , the matrix for the uncompressed kernel, because the kernel is a string of repeated bytes.To produce a CRC checksum value from a matrix, multiply the matrix by the zero vector.The zero vector in homogeneous coordinates, that is: 32 0’s followed by a 1.
Here we omit the minor complication of pre – and post-conditioning the checksum.To compute the checksum for every file, we work backwards.Continue the procedure, accumulating state change matrices into M , until all the files have been processed.Earlier we hit a wall on expansion due to limits of the zip format—it was impossible to produce more than about TB of output, no matter how cleverly packed the zip file.It is possible to surpass those limits using Zip64 , an extension to the zip format that increases the size of certain header fields to 64 bits.
Support for Zip64 is by no means universal , but it is one of the more commonly implemented extensions.As regards the compression ratio, the effect of Zip64 is to increase the size of a central directory header from 46 bytes to 58 bytes, and the size of a local directory header from 30 bytes to 50 bytes.Referring to the formula for optimal expansion in the simplified model, we see that a zip bomb in Zip64 format still grows quadratically, but more slowly because of the larger denominator—this is visible in the figure below in the Zip64 line’s slightly lower vertical placement.
In exchange for the loss of compatibility and slower growth, we get the removal of all practical file size limits.Suppose we want a zip bomb that expands to 4.How big must the zip file be? Using binary search, we find that the smallest zip file whose unzipped size exceeds the unzipped size of With Zip64, it’s no longer practically interesting to consider the maximum compression ratio, because we can just keep increasing the zip file size, and the compression ratio along with it, until even the compressed zip file is prohibitively large.
An interesting threshold, though, is 2 64 bytes 18 EB or 16 EiB — that much data will not fit on most filesystems.Binary search finds the smallest zip bomb that produces at least that much output: it contains 12 million files and has a compressed kernel of 1.The total size of the zip file is 2.I didn’t make this one downloadable, but you can generate it yourself using the source code.
Probably the second most common algorithm is bzip2 , while not as compatible as DEFLATE, is probably the second most commonly supported compression algorithm.Authors are encouraged to avoid using the u element where it could be confused for a hyperlink.In this example, a u element is used to mark a word as misspelt:.
HTML element represents text which is marked or highlighted for reference or notation purposes, due to the marked passage’s relevance or importance in the enclosing context.The mark element represents a run of text in one document marked or highlighted for reference purposes, due to its relevance in another context.When used in a quotation or other block of text referred to from the prose, it indicates a highlight that was not originally present but which has been added to bring the reader’s attention to a part of the text that might not have been considered important by the original author when the block was originally written, but which is now under previously unexpected scrutiny.
When used in the main prose of a document, it indicates a part of the document that has been highlighted due to its likely relevance to the user’s current activity.This example shows how the mark element can be used to bring attention to a particular part of a quotation:.
If the goal was to mark the element as misspelt, however, the u element, possibly with a class, would be more appropriate.
Another example of the mark element is highlighting parts of a document that are matching some search string.If someone looked at a document, and the server knew that the user was searching for the word “kitten”, then the server might return the document with one paragraph modified as follows:.
This is separate from syntax highlighting , for which span is more appropriate.Combining both, one would get:.This is another example showing the use of mark to highlight a part of quoted text that was originally not emphasized.
In this example, common typographic conventions have led the author to explicitly style mark elements in quotes to render in italics.Note, incidentally, the distinction between the em element in this example, which is part of the original text being quoted, and the mark element, which is highlighting a part for comment.
The following example shows the difference between denoting the importance of a span of text strong as opposed to denoting the relevance of a span of text mark.It is an extract from a textbook, where the extract has had the parts relevant to the exam highlighted.
The safety warnings, important though they may be, are apparently not relevant to the exam.HTML element tells the browser’s bidirectional algorithm to treat the text it contains in isolation from its surrounding text.
It’s particularly useful when a website dynamically inserts some text and doesn’t know the directionality of the text being inserted.Content attributes : Global attributes Also, the dir global attribute has special semantics on this element.The bdi element represents a span of text that is to be isolated from its surroundings for the purposes of bidirectional text formatting.
The dir global attribute defaults to auto on this element it never inherits from the parent element like with other elements.This element has rendering requirements involving the bidirectional algorithm.This element is especially useful when embedding user-generated content with an unknown directionality.
In this example, usernames are shown along with the number of posts that the user has submitted.If the bdi element were not used, the username of the Arabic user would end up confusing the text the bidirectional algorithm would put the colon and the number “3” next to the word “User” rather than next to the word “posts”.
HTML element overrides the current directionality of text, so that the text within is rendered in a different direction.The bdo element represents explicit text directionality formatting control for its children.
It allows authors to override the Unicode bidirectional algorithm by explicitly specifying a direction override.Authors must specify the dir attribute on this element, with the value ltr to specify a left-to-right override and with the value rtl to specify a right-to-left override.The auto value must not be specified.HTML element is a generic inline container for phrasing content, which does not inherently represent anything.
It can be used to group elements for styling purposes using the class or id attributes , or because they share attribute values, such as lang.
It should be used only when no other semantic element is appropriate.In this example, a code fragment is marked up using span elements and class attributes so that its keywords and identifiers can be color-coded from CSS:.HTML element produces a line break in text carriage-return.It is useful for writing a poem or an address, where the division of lines is significant.
Content model : Nothing.While line breaks are usually represented in visual media by physically moving subsequent text to a new line, a style sheet or user agent would be equally justified in causing line breaks to be rendered in a different manner, for instance as green dots, or as extra spacing.
The following example is correct usage of the br element:.The following examples are non-conforming, as they abuse the br element:.If a paragraph consists of nothing but a single br element, it represents a placeholder blank line e.Such blank lines must not be used for presentation purposes.Any content inside br elements must not be considered part of the surrounding text.
HTML element represents a word break opportunity—a position within text where the browser may optionally break a line, though its line-breaking rules would not otherwise create a break at that location.Categories : Flow content.
The wbr element represents a line break opportunity.In the following example, someone is quoted as saying something which, for effect, is written as one long word.
However, to ensure that the text can be wrapped in a readable fashion, the individual words in the quote are separated using a wbr element.Any content inside wbr elements must not be considered part of the surrounding text.Links are a conceptual construct, created by a , area , form , and link elements, that represent a connection between two resources, one of which is the current Document.There are two kinds of links in HTML:.These are links to resources that are to be used to augment the current document, generally automatically processed by the user agent.
All external resource links have a fetch and process the linked resource algorithm which describes how the resource is obtained.Hyperlinks These are links to other resources that are generally exposed to the user by the user agent so that the user can cause the user agent to navigate to those resources, e.For link elements with an href attribute and a rel attribute, links must be created for the keywords of the rel attribute, as defined for those keywords in the link types section.
Similarly, for a and area elements with an href attribute and a rel attribute, links must be created for the keywords of the rel attribute as defined for those keywords in the link types section.Unlike link elements, however, a and area elements with an href attribute that either do not have a rel attribute, or whose rel attribute has no keywords that are defined as specifying hyperlinks , must also create a hyperlink.This implied hyperlink has no special meaning it has no link type beyond linking the element’s node document to the resource given by the element’s href attribute.
Similarly, for form elements with a rel attribute, links must be created for the keywords of the rel attribute as defined for those keywords in the link types section.A hyperlink can have one or more hyperlink annotations that modify the processing semantics of that hyperlink.The href attribute on a and area elements must have a value that is a valid URL potentially surrounded by spaces.
The href attribute on a and area elements is not required; when those elements do not have href attributes they do not create hyperlinks.The target attribute, if present, must be a valid browsing context name or keyword.It gives the name of the browsing context that will be used.User agents use this name when following hyperlinks.When an a or area element’s activation behavior is invoked, the user agent may allow the user to indicate a preference regarding whether the hyperlink is to be used for navigation or whether the resource it specifies is to be downloaded.
In the absence of a user preference, the default should be navigation if the element has no download attribute, and should be to download the specified resource if it does.
Whether determined by the user’s preferences or via the presence or absence of the attribute, if the decision is to use the hyperlink for navigation then the user agent must follow the hyperlink , and if the decision is to use the hyperlink to download a resource, the user agent must download the hyperlink.These terms are defined in subsequent sections below.
The download attribute, if present, indicates that the author intends the hyperlink to be used for downloading a resource.The attribute may have a value; the value, if any, specifies the default filename that the author recommends for use in labeling the resource in a local file system.There are no restrictions on allowed values, but authors are cautioned that most file systems have limitations with regard to what punctuation is supported in filenames, and user agents are likely to adjust filenames accordingly.
The value is used by the user agent for hyperlink auditing.The rel attribute on a and area elements controls what kinds of links the elements create.The attribute’s value must be an unordered set of unique space-separated tokens.The allowed keywords and their meanings are defined below.The possible supported tokens are noreferrer , noopener , and opener.The rel attribute has no default value.If the attribute is omitted or if none of the values in the attribute are recognized by the user agent, then the document has no particular relationship with the destination resource other than there being a hyperlink between the two.
The hreflang attribute on a elements that create hyperlinks , if present, gives the language of the linked resource.It is purely advisory.The value must be a valid BCP 47 language tag.The type attribute, if present, gives the MIME type of the linked resource.The value must be a valid MIME type string.User agents must not consider the type attribute authoritative — upon fetching the resource, user agents must not use metadata included in the link to the resource to determine its type.
The referrerpolicy attribute is a referrer policy attribute.Its purpose is to set the referrer policy used when following hyperlinks.It is initially null.If this element’s href content attribute is absent, set this element’s url to null.Otherwise, parse this element’s href content attribute value relative to this element’s node document.If parsing is successful, set this element’s url to the result; otherwise, set this element’s url to null.
When elements implementing the HTMLHyperlinkElementUtils mixin are created, and whenever those elements have their href content attribute set, changed, or removed, the user agent must set the url.If element’s url is non-null, its scheme is ” blob “, and it has an opaque path , then terminate these steps.Set the url.To update href , set the element’s href content attribute’s value to the element’s url , serialized.The href getter steps are:.
Reinitialize url.Let url be this ‘s url.If url is null and this has no href content attribute, return the empty string.Otherwise, if url is null, return this ‘s href content attribute’s value.
Return url , serialized.The href setter steps are to set this ‘s href content attribute’s value to the given value.The origin getter steps are:.If this ‘s url is null, return the empty string.Return the serialization of this ‘s url ‘s origin.
The protocol getter steps are:.If this ‘s url is null, return ” : “.Return this ‘s url ‘s scheme , followed by ” : “.The protocol setter steps are:.If this ‘s url is null, then return.
Basic URL parse the given value, followed by ” : “, with this ‘s url as url and scheme start state as state override.
Because the URL parser ignores multiple consecutive colons, providing a value of ” https: ” or even ” https ” is the same as providing a value of ” https “.Update href.The username getter steps are:.Return this ‘s url ‘s username.
The username setter steps are:.Set the username , given url and the given value.The password getter steps are:.If url is null, then return the empty string.Return url ‘s password.The password setter steps are:.Set the password , given url and the given value.The host getter steps are:.If url or url ‘s host is null, return the empty string.If url ‘s port is null, return url ‘s host , serialized.Return url ‘s host , serialized , followed by ” : ” and url ‘s port , serialized.
The host setter steps are:.If url is null or url has an opaque path , then return.Basic URL parse the given value, with url as url and host state as state override.The hostname getter steps are:.Return url ‘s host , serialized.The hostname setter steps are:.Basic URL parse the given value, with url as url and hostname state as state override.
The port getter steps are:.If url or url ‘s port is null, return the empty string.Return url ‘s port , serialized.The port setter steps are:.If the given value is the empty string, then set url ‘s port to null.Otherwise, basic URL parse the given value, with url as url and port state as state override.The pathname getter steps are:.Return the result of URL path serializing url.The pathname setter steps are:.
Set url ‘s path to the empty list.Basic URL parse the given value, with url as url and path start state as state override.The search getter steps are:.If url is null, or url ‘s query is either null or the empty string, return the empty string.Return “? The search setter steps are:.If url is null, terminate these steps.If the given value is the empty string, set url ‘s query to null.Let input be the given value with a single leading “?
Set url ‘s query to the empty string.Basic URL parse input , with null, this element’s node document ‘s document’s character encoding , url as url , and query state as state override.The hash getter steps are:.If url is null, or url ‘s fragment is either null or the empty string, return the empty string.
Return ” “, followed by url ‘s fragment.The hash setter steps are:.If url is null, then return.If the given value is the empty string, set url ‘s fragment to null.Let input be the given value with a single leading ” ” removed, if any.Set url ‘s fragment to the empty string.Basic URL parse input , with url as url and fragment state as state override.This is also used by form submission for the form element.The exception for a elements is for compatibility with web content.
To get an element’s noopener , given an a , area , or form element element and a string target :.If element ‘s link types include the noopener or noreferrer keyword, then return true.Return false.To follow the hyperlink created by an element subject , given an optional hyperlinkSuffix default null :.If subject cannot navigate , then return.Let replace be false.
Let source be subject ‘s node document ‘s browsing context.Let targetAttributeValue be the empty string.If subject is an a or area element, then set targetAttributeValue to the result of getting an element’s target given subject.
Let noopener be the result of getting an element’s noopener with subject and targetAttributeValue.Let target and windowType be the result of applying the rules for choosing a browsing context given targetAttributeValue , source , and noopener.If target is null, then return.
Parse a URL given subject ‘s href attribute, relative to subject ‘s node document.Otherwise, if parsing the URL failed, the user agent may report the error to the user in a user-agent-specific manner, may queue an element task on the DOM manipulation task source given subject to navigate the target browsing context to an error page to report the error, or may ignore the error and do nothing.In any case, the user agent must then return.If hyperlinkSuffix is non-null, then append it to URL.
Let request be a new request whose URL is URL and whose referrer policy is the current state of subject ‘s referrerpolicy content attribute.If subject ‘s link types includes the noreferrer keyword, then set request ‘s referrer to ” no-referrer “.Let historyHandling be ” replace ” if windowType is not ” existing or none “; otherwise, ” default “.
Unlike many other types of navigations, following hyperlinks does not have special ” replace ” behavior for when documents are not completely loaded.This is true for both user-initiated instances of following hyperlinks, as well as script-triggered ones via, e.
Queue an element task on the DOM manipulation task source given subject to navigate target to request with historyHandling set to historyHandling and the source browsing context set to source.To indicate that a resource is intended to be downloaded for use later, rather than immediately used, the download attribute can be specified on the a or area element that creates the hyperlink to that resource.
The attribute can furthermore be given a value, to specify the filename that user agents are to use when storing the resource in a file system.This is to protect users from being made to download sensitive personal or confidential information without their full understanding.
The following allowed to download algorithm takes an initiator browsing context and an instantiator browsing context , and returns a boolean indicating whether or not downloading is allowed:.If the initiator browsing context ‘s sandboxing flags has the sandboxed downloads browsing context flag set, then return false.
If the instantiator browsing context is non-null, and its sandboxing flags has the sandboxed downloads browsing context flag set, then return false.Optionally, the user agent may return false, if it believes doing so would safeguard the user from a potentially hostile download.
Return true.To download the hyperlink created by an element subject , given an optional hyperlinkSuffix default null :.Run the allowed to download algorithm with the subject ‘s node document ‘s browsing context and null.
If the algorithm returns false, then return.If parsing the URL fails, the user agent may report the error to the user in a user-agent-specific manner, may navigate to an error page to report the error, or may ignore the error and do nothing.In either case, the user agent must return.Run these steps in parallel :.Let request be a new request whose URL is URL , client is entry settings object , initiator is ” download “, destination is the empty string, and whose synchronous flag and use-URL-credentials flag are set.
Handle the result of fetching request as a download.When a user agent is to handle a resource obtained from a fetch as a download , it should provide the user with a way to save the resource for later use, if a resource is successfully obtained.Otherwise, it should report any problems downloading the file to the user.If the user agent needs a filename for a resource being handled as a download , it should select one using the following algorithm.
This algorithm is intended to mitigate security dangers involved in downloading files from untrusted sites, and user agents are strongly urged to follow it.Let filename be the undefined value.Let resource origin be the origin of the URL of the resource being downloaded, unless that URL’s scheme component is data , in which case let resource origin be the same as the interface origin , if any.If there is no interface origin , then let trusted operation be true.
Otherwise, let trusted operation be true if resource origin is the same origin as interface origin , and false otherwise.Let proposed filename have the value of the download attribute of the element of the hyperlink that initiated the download at the time the download was initiated.If trusted operation is true, let filename have the value of proposed filename , and jump to the step labeled sanitize below.
Let filename be set to the user’s preferred filename or to a filename selected by the user agent, and jump to the step labeled sanitize below.If the algorithm reaches this step, then a download was begun from a different origin than the resource being downloaded, and the origin did not mark the file as suitable for downloading, and the download was not initiated by the user.
This could be because a download attribute was used to trigger the download, or because the resource in question is not of a type that the user agent supports.
This could be dangerous, because, for instance, a hostile server could be trying to get a user to unknowingly download private information and then re-upload it to the hostile server, by tricking the user into thinking the data is from the hostile server.Thus, it is in the user’s interests that the user be somehow notified that the resource in question comes from quite a different source, and to prevent confusion, any suggested filename from the potentially hostile interface origin should be ignored.
Sanitize : Optionally, allow the user to influence filename.For example, a user agent could prompt the user for a filename, potentially providing the value of filename as determined above as a default value.Adjust filename to be suitable for the local file system.For example, this could involve removing characters that are not legal in filenames, or trimming leading and trailing whitespace.
If the platform conventions do not in any way use extensions to determine the types of file on the file system, then return filename as the filename.Let claimed type be the type given by the resource’s Content-Type metadata , if any is known.Let named type be the type given by filename ‘s extension , if any is known.
For the purposes of this step, a type is a mapping of a MIME type to an extension.If named type is consistent with the user’s preferences e.
If claimed type and named type are the same type i.If the claimed type is known, then alter filename to add an extension corresponding to claimed type.Otherwise, if named type is known to be potentially dangerous e.This last step would make it impossible to download executables, which might not be desirable.As always, implementers are forced to balance security and usability in this matter.
Return filename as the filename.For the purposes of this algorithm, a file extension consists of any part of the filename that platform conventions dictate will be used for identifying the type of the file.For example, many operating systems use the part of the filename following the last dot “.
User agents should ignore any directory or path information provided by the resource itself, its URL , and any download attribute, in deciding where to store the resulting file in the user’s file system.
If a hyperlink created by an a or area element has a ping attribute, and the user follows the hyperlink, and the value of the element’s href attribute can be parsed , relative to the element’s node document , without failure, then the user agent must take the ping attribute’s value, split that string on ASCII whitespace , parse each resulting token relative to the element’s node document , and then run these steps for each resulting URL record ping URL , ignoring tokens that fail to parse:.
Optionally, return.For example, the user agent might wish to ignore any or all ping URLs in accordance with the user’s expressed preferences.Fetch request.This may be done in parallel with the primary fetch, and is independent of the result of that fetch.Based on the user’s preferences, UAs may either ignore the ping attribute altogether, or selectively ignore URLs in the list e.
User agents must ignore any entity bodies returned in the responses.User agents may close the connection prematurely once they start receiving a response body.
When the ping attribute is present, user agents should clearly indicate to the user that following the hyperlink will also cause secondary requests to be sent in the background, possibly including listing the actual target URLs.
However, the ping attribute provides these advantages to the user over those alternatives:.Thus, while it is possible to track users without this feature, authors are encouraged to use the ping attribute so that the user agent can make the user experience more transparent.Their value is a URL , serialized.This table is non-normative; the actual definitions for the link types are given in the next few sections.In this section, the term referenced document refers to the resource identified by the element representing the link, and the term current document refers to the resource within which the element representing the link finds itself.
To determine which link types apply to a link , a , area , or form element, the element’s rel attribute must be split on ASCII whitespace.The resulting tokens are the keywords for the link types that apply to that element.Except where otherwise specified, a keyword must not be specified more than once per rel attribute.
Some of the sections that follow the table below list synonyms for certain keywords.The indicated synonyms are to be handled as specified by user agents, but must not be used in documents for example, the keyword ” copyright “.
Keywords that are body-ok affect whether link elements are allowed in the body.The body-ok keywords are dns-prefetch , modulepreload , pingback , preconnect , prefetch , preload , prerender , and stylesheet.New link types that are to be implemented by web browsers are to be added to this standard.
The remainder can be registered as extensions.Chrome Android? WebView Android? Samsung Internet? Opera Android? The alternate keyword may be used with link , a , and area elements.The alternate keyword modifies the meaning of the stylesheet keyword in the way described for that keyword.The alternate keyword does not create a link of its own.Here, a set of link elements provide some style sheets:.The keyword creates a hyperlink referencing a syndication feed though not necessarily syndicating exactly the same content as the current page.
If the user agent has the concept of a default syndication feed, the first such element in tree order should be used as the default.The following link elements give syndication feeds for a blog:.Such link elements would be used by user agents engaged in feed autodiscovery, with the first being the default where applicable.
The following example offers various different syndication feeds to the user, using a elements:.The keyword creates a hyperlink referencing an alternate representation of the current document.The nature of the referenced document is given by the hreflang , and type attributes.
If the alternate keyword is used with the hreflang attribute, and that attribute’s value differs from the document element ‘s language , it indicates that the referenced document is a translation.If the alternate keyword is used with the type attribute, it indicates that the referenced document is a reformulation of the current document in the specified format.
The hreflang and type attributes can be combined when specified with the alternate keyword.The following example shows how you can specify versions of the page that use alternative formats, are aimed at other languages, and that are intended for other media:.
This relationship is transitive — that is, if a document links to two other documents with the link type ” alternate “, then, in addition to implying that those documents are alternative representations of the first document, it is also implying that those two documents are alternative representations of each other.
The author keyword may be used with link , a , and area elements.This keyword creates a hyperlink.For a and area elements, the author keyword indicates that the referenced document provides further information about the author of the nearest article element ancestor of the element defining the hyperlink, if there is one, or of the page as a whole, otherwise.
For link elements, the author keyword indicates that the referenced document provides further information about the author for the page as a whole.The “referenced document” can be, and often is, a mailto: URL giving the email address of the author.
Synonyms : For historical reasons, user agents must also treat link , a , and area elements that have a rev attribute with the value ” made ” as having the author keyword specified as a link relationship.The bookmark keyword may be used with a and area elements.The bookmark keyword gives a permalink for the nearest ancestor article element of the linking element in question, or of the section the linking element is most closely associated with , if there are no ancestor article elements.
The following snippet has three permalinks.A user agent could determine which permalink applies to which part of the spec by looking at where the permalinks are given.The canonical keyword may be used with link element.That helps search engines reduce duplicate content, as described in more detail in The Canonical Link Relation.
The dns-prefetch keyword may be used with link elements.This keyword creates an external resource link.This keyword is body-ok.The dns-prefetch keyword indicates that preemptively performing DNS resolution for the origin of the specified resource is likely to be beneficial, as it is highly likely that the user will require resources located at that origin , and the user experience would be improved by preempting the latency costs associated with DNS resolution.
User agents must implement the processing model of the dns-prefetch keyword described in Resource Hints.There is no default type for resources given by the dns-prefetch keyword.The external keyword may be used with a , area , and form elements.This keyword does not create a hyperlink , but annotates any other hyperlinks created by the element the implied hyperlink, if no other keywords create one.The external keyword indicates that the link is leading to a document that is not part of the site that the current document forms a part of.
The help keyword may be used with link , a , area , and form elements.For a , area , and form elements, the help keyword indicates that the referenced document provides further help information for the parent of the element defining the hyperlink, and its children.In the following example, the form control has associated context-sensitive help.
The user agent could use this information, for example, displaying the referenced document if the user presses the “Help” or “F1” key.For link elements, the help keyword indicates that the referenced document provides help for the page as a whole.
For a and area elements, on some browsers, the help keyword causes the link to use a different cursor.The specified resource is an icon representing the page or site, and should be used by the user agent when representing the page in the user interface.
Icons could be auditory icons, visual icons, or other kinds of icons.If multiple icons are provided, the user agent must select the most appropriate icon according to the type , media , and sizes attributes.If there are multiple equally appropriate icons, user agents must use the last one declared in tree order at the time that the user agent collected the list of icons.
If the user agent tries to use an icon but that icon is determined, upon closer examination, to in fact be inappropriate e.User agents are not required to update icons when the list of icons changes, but are encouraged to do so.There is no default type for resources given by the icon keyword.However, for the purposes of determining the type of the resource , user agents must expect the resource to be an image.
The sizes keywords represent icon sizes in raw pixels as opposed to CSS pixels.An icon that is 50 CSS pixels wide intended for displays with a device pixel density of two device pixels per CSS pixel 2x, dpi would have a width of raw pixels.This feature does not support indicating that a different resource is to be used for small high-resolution icons vs large low-resolution icons e.
To parse and process the attribute’s value, the user agent must first split the attribute’s value on ASCII whitespace , and must then parse each resulting keyword to determine what it represents.
The any keyword represents that the resource contains a scalable icon, e.Return for that keyword.Let width string be the string before the ” x ” or ” X “.
Let height string be the string after the ” x ” or ” X “.Apply the rules for parsing non-negative integers to width string to obtain width.Apply the rules for parsing non-negative integers to height string to obtain height.The keyword represents that the resource contains a bitmap icon with a width of width device pixels and a height of height device pixels.The keywords specified on the sizes attribute must not represent icon sizes that are not actually available in the linked resource.
The linked resource fetch setup steps for this type of linked resource, given a link element el and request request , are:.Set request ‘s destination to ” image “.Let response be the result of fetching request.Use response ‘s unsafe response as an icon as if it had been declared using the icon keyword.The following snippet shows the top part of an application with several icons.For historical reasons, the icon keyword may be preceded by the keyword ” shortcut “.
The license keyword may be used with link , a , area , and form elements.The license keyword indicates that the referenced document provides the copyright license terms under which the main content of the current document is provided.This specification does not specify how to distinguish between the main content of a document and content that is not deemed to be part of that main content.The distinction should be made clear to the user.Consider a photo sharing site.A page on that site might describe and show a photograph, and the page might be marked up as follows:.
In this case the license applies to just the photo the main content of the document , not the whole document.In particular not the design of the page itself, which is covered by the copyright given at the bottom of the document.This could be made clearer in the styling e.Synonyms : For historical reasons, user agents must also treat the keyword ” copyright ” like the license keyword.Firefox Android?
Safari iOS? The manifest keyword may be used with link elements.The manifest keyword indicates the manifest file that provides metadata associated with the current document.There is no default type for resources given by the manifest keyword.
When a web application is not installed , the appropriate time to fetch and process the linked resource for this link type is when the user agent deems it necessary.For example, when the user chooses to install the web application.
For an installed web application , the appropriate times to fetch and process the linked resource for this link type are:.When the external resource link is created on a link element that is already browsing-context connected.
When the external resource link ‘s link element becomes browsing-context connected.When the href attribute of the link element of an external resource link that is already browsing-context connected is changed.In either cases, only the first link element in tree order whose rel attribute contains the token manifest may be used.
A user agent must not delay the load event for this link type.Let context be el ‘s node document ‘s browsing context.If context is null, then return false.If context is not a top-level browsing context , then return false.
Set request ‘s initiator to ” manifest “.Set request ‘s destination to ” manifest “.Set request ‘s mode to ” cors “.Set request ‘s credentials mode to the CORS settings attribute credentials mode for el ‘s crossorigin content attribute.To process this type of linked resource given a link element el , boolean success , and response response :.If success is true, then process the manifest given el and response.
The modulepreload keyword is a specialized alternative to the preload keyword, with a processing model geared toward preloading module scripts.In particular, it uses the specific fetch behavior for module scripts including, e.In contrast, a similar external resource link using the preload keyword would place the result in the preload cache, without affecting the document’s module map.
Additionally, implementations can take advantage of the fact that module scripts declare their dependencies in order to fetch the specified module’s dependency as well.This is intended as an optimization opportunity, since the user agent knows that, in all likelihood, those dependencies will also be needed later.It will not generally be observable without using technology such as service workers, or monitoring on the server side.Notably, the appropriate load or error events will occur after the specified module is fetched, and will not wait for any dependencies.
The appropriate times to fetch and process the linked resource for such a link are:.Unlike some other link relations, changing the relevant attributes such as as , crossorigin , and referrerpolicy of such a link does not trigger a new fetch.
This is because the document’s module map has already been populated by a previous fetch, and so re-fetching would be pointless.The fetch and process the linked resource algorithm for modulepreload links, given a link element el , is as follows:.If el ‘s href attribute’s value is the empty string, then return.
Let destination be the current state of the as attribute a destination , or ” script ” if it is in no state.
If destination is not script-like , then queue an element task on the networking task source given the link element to fire an event named error at the link element, and return.Parse a URL given el ‘s href attribute, relative to the element’s node document.If that fails, then return.Otherwise, let url be the resulting URL record.
Let settings object be the link element’s node document ‘s relevant settings object.Let credentials mode be the CORS settings attribute credentials mode for the crossorigin attribute.Let cryptographic nonce be the current value of the element’s [[CryptographicNonce]] internal slot.
Let integrity metadata be the value of the integrity attribute, if it is specified, or the empty string otherwise.Let referrer policy be the current state of the element’s referrerpolicy attribute.Let options be a script fetch options whose cryptographic nonce is cryptographic nonce , integrity metadata is integrity metadata , parser metadata is ” not-parser-inserted “, credentials mode is credentials mode , and referrer policy is referrer policy.
Fetch a modulepreload module script graph given url , destination , settings object , and options.Wait until the algorithm asynchronously completes with result.
If result is null, then fire an event named error at the link element, and return.Fire an event named load at the link element.The following snippet shows the top part of an application with several modules preloaded:.Here we see the application developer has used modulepreload to declare all of the modules in their module graph, ensuring that the user agent initiates fetches for them all.
Without such preloading, the user agent might need to go through multiple network roundtrips before discovering helpers.In this way, modulepreload link elements can be used as a sort of “manifest” of the application’s modules.The following code shows how modulepreload links can be used in conjunction with import to ensure network fetching is done ahead of time, so that when import is called, the module is already ready but not evaluated in the module map :.
The nofollow keyword may be used with a , area , and form elements.The nofollow keyword indicates that the link is not endorsed by the original author or publisher of the page, or that the link to the referenced document was included primarily because of a commercial relationship between people affiliated with the two pages.
The keyword indicates that any newly created top-level browsing context which results from following the hyperlink will not be an auxiliary browsing context.See also the processing model where the branching between an auxiliary browsing context and a top-level browsing context is defined.
This typically creates an auxiliary browsing context assuming there is no existing browsing context whose browsing context name is ” example ” :.This creates a top-level browsing context that is not an auxiliary browsing context assuming the same thing :.
These are equivalent and only navigate the parent browsing context :.It indicates that no referrer information is to be leaked when following the link and also implies the noopener keyword behavior under the same conditions.
See also the processing model where referrer is directly manipulated.The opener keyword may be used with a , area , and form elements.The keyword indicates that any newly created top-level browsing context which results from following the hyperlink will be an auxiliary browsing context.
In the following example the opener is used to allow the help page popup to navigate its opener, e.The pingback keyword may be used with link elements.For the semantics of the pingback keyword, see Pingback 1.The preconnect keyword indicates that preemptively initiating a connection to the origin of the specified resource is likely to be beneficial, as it is highly likely that the user will require resources located at that origin , and the user experience would be improved by preempting the latency costs associated with establishing the connection.
User agents must implement the processing model of the preconnect keyword described in Resource Hints.There is no default type for resources given by the preconnect keyword.
The prefetch keyword indicates that preemptively fetching and caching the specified resource is likely to be beneficial, as it is highly likely that the user will require this resource for future navigations.User agents must implement the processing model of the prefetch keyword described in Resource Hints.There is no default type for resources given by the prefetch keyword.This ensures they are available earlier and are less likely to block the page’s render, improving performance.
The preload keyword may be used with link elements.The preload keyword indicates that the user agent will preemptively fetch and cache the specified resource according to the potential destination given by the as attribute and the priority associated with the corresponding destination , as it is highly likely that the user will require this resource for the current navigation.
There is no default type for resources given by the preload keyword.A Document has a map of preloaded resources , which is a map , initially empty.A preload key is a struct.It has the following items :.A preload entry is a struct.To consume a preloaded resource for Window window , given a URL url , a string destination , a string mode , a string credentialsMode , a string integrityMetadata , and onResponseAvailable , which is an algorithm accepting a response :.
Let key be a preload key whose URL is url , destination is destination , integrity metadata is integrityMetadata , mode is mode , and credentials mode is credentialsMode.Let preloads be window ‘s associated Document ‘s map of preloaded resources.If key does not exist in preloads , then return false.
Let entry be preloads [ key ].Remove preloads [ key ].If entry response is null, then set entry ‘s on response available to onResponseAvailable.Otherwise, call onResponseAvailable with entry ‘s response.The fetch and process the linked resource steps for this type of linked resource, given a link element el , are:.
Let as be the current state of el ‘s as attribute.If as does not represent a state, return false.Let request be the result of creating a link element request given el and the result of translating as.
If request is null, then return.If as is ” image “, then:.Let selected source and selected pixel density be the URL and pixel density that results from selecting an image source given el , respectively.If selected source is null, then return false.Parse selected source , relative to el ‘s node document.If that fails, then return false.
Set request ‘s URL to url.Let preloadKey be a preload key whose URL is request ‘s URL , destination is request ‘s destination , integrity metadata is request ‘s integrity metadata , mode is request ‘s mode , and credentials mode is request ‘s credentials mode.Let preloadEntry be a new preload entry.Set el ‘s node document ‘s map of preloaded resources [ preloadKey ] to preloadEntry.Fetch request , with processResponseConsumeBody set to the following steps given response response and null or byte sequence bytesOrNull :.
If bytesOrNull is a byte sequence , then set response ‘s body to the first return value of safely extracting bytesOrNull.By using processResponseConsumeBody , we have extracted the entire body.This is necessary to ensure the preloader loads the entire body from the network, regardless of whether the preload will be consumed which is uncertain at this point.
This step then resets the request’s body to a new body containing the same bytes, so that other specifications can read from it at the time of actual consumption, despite us having already done so once.Otherwise, set response to a network error.Finalize and report timing with response , given el ‘s relevant global object and ” link “.Fire an event named load at el.
If preloadEntry ‘s on response available is null, then set preloadEntry ‘s response to response.Otherwise, call preloadEntry ‘s on response available with response.
The prerender keyword indicates that the specified resource might be required by the next navigation, and so it may be beneficial to not only preemptively fetch the resource, but also to process it, e.User agents must implement the processing model of the prerender keyword described in Resource Hints.There is no default type for resources given by the prerender keyword.The search keyword may be used with link , a , area , and form elements.
The search keyword indicates that the referenced document provides an interface specifically for searching the document and its related resources.OpenSearch description documents can be used with link elements and the search link type to enable user agents to autodiscover search interfaces.The stylesheet keyword may be used with link elements.This keyword creates an external resource link that contributes to the styling processing model.
The specified resource is a CSS style sheet that describes how to present the document.If the alternate keyword is also specified on the link element, then the link is an alternative style sheet ; in this case, the title attribute must be specified on the link element, with a non-empty value.
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- Let response be the result of fetching request.
- Sales Representative.
- There is no default type for resources given by the icon keyword.
- But there are also non-compressed blocks, which start with a 5-byte header with a length field that means simply, “output the next n bytes verbatim.
- The reference may be in an abbreviated form according to context-appropriate conventions related to citation metadata.
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An rp element whose parent element is not a ruby element represents its children.The exception for a elements is for compatibility with web content.
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- The output is the concatenation of decompressing all the blocks in order.
- In other cases, if an element is really needed, the span element can be used.
- This is distinct from a sidebar, which might be multiple paragraphs long and is removed from the main flow of text.
- The href getter steps are:.
- It is compatible with most zip parsers, the exceptions being “streaming” parsers that parse in one pass without first consulting the zip file’s central directory.
- There is nothing wrong with attempting to detect and reject certain constructions as a first-pass optimization, but you can’t stop there.
- Contexts in which this element can be used : Where flow content is expected, but only if it is a hierarchically correct main element.
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: To update hrefset the element’s href content attribute’s value to the element’s urlserialized.
Instagram bot in Node.The value must be a valid BCP 47 language tag.Zip bombs often rely on repetition of identical files to achieve their extreme compression ratios.
- This ensures they are available earlier and are less likely to block the page’s render, improving performance.
- The most likely outcome is that they will extract only the first file.
- Set saved start index to the value of start index.
- The q element must not be used in place of quotation marks that do not represent quotes; for example, it is inappropriate to use the q element for marking up sarcastic statements.
- This could be made clearer in the styling e.
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