A document that uses polyglot markup is an HTML5 document which is at the same time an XML document and an HTML document, and which meets a well defined set of constraints. Polyglot markup that meets these constraints as interpreted as compatible, regardless of whether they are processed as HTML or as XHTML, per the HTML5 specification. Polyglot markup uses a specific doctype, namespace declarations, and a specific case—normally lower case but occasionally camel case—for element and attribute names. Polyglot markup uses lower case for certain attribute values. Further constraints include those on empty elements, named entity references, and the use of scripts and style.
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This document summarizes design guidelines for authors who wish their XHTML or HTML documents to validate on either HTML or XML parsers, assuming the parsers to be HTML5-compliant. This specification is intended to be used by web authors. It is not a specification for user agents and creates no obligations on user agents. Note that this recommendation does not define how HTML5-conforming user agents should process HTML documents. Nor does it define the meaning of the Internet Media Type text/html. For user agent guidance and for these definitions, see [HTML5] and [RFC2854].
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Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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This section is non-normative.
It is often valuable to be able to serve HTML5 documents that are also valid XML documents. An author may, for example, use XML tools to generate a document, and they and others may process the document using XML tools. These documents are served as text/html. The language used to create documents that can be parsed by both HTML and XML parsers is called polyglot markup. Polyglot markup is the overlap language of documents which are both HTML5 documents and XML documents.
Polyglot markup does not use processing instructions. Note that the parsing rules for the XML declaration are not processing instructions and are defined separately in Prolog and Document Type Declaration.
Polyglot markup uses either UTF-8 or UTF-16, although generally UTF-8 is preferred. When polyglot markup uses UTF-16, it should include the BOM indicating UTF-16LE or UTF-16BE. In addition, polyglot markup need not include the meta charset declaration, because the parser would have to read UTF-16 in order to parse it by definition.
In short, for correct character encoding, polyglot markup must either:
metatag to specify the appropriate character encoding.
If polyglot markup uses an encoding other than UTF-8 or UTF-16, it must include the XML declaration;
however, in this case the document must
also include the HTML
meta tag specifying the character
When polyglot markup uses both the XML declaration and the HTML
tag, these must specify the same
character and coding.
Polyglot markup uses the
<!DOCTYPE html> doctype.
Note that for polyglot markup the string,
html, must be lower case.
For a pure HTML document, the string is defined as case-insensitive. [HTML5]
The following rules apply to namespaces used in polyglot markup.
<html>element uses the namespace declaration
<math>elements uses the namespace declaration
<svg>elements uses the namespace declaration
xmlns:xlink="http://www.w3.org/1999/xlink"before using xlink:href. The prefix can be defined either:
<svg>element that contains one or more elements with xlink:href attributes.
Each document using polyglot markup must have a root
html element must contain both a
head and a
head element must
Polyglot markup must
explicitly have a
tbody element surrounding groups of
elements within a
HTML parsers insert the
tbody element, but XML
parsers do not, thus creating different DOMs.
<table> <tbody> <tr>...Incorrect:
The following guidelines apply to any usage of element names, attribute names, or attribute values in markup, script, or CSS. When required, polyglot markup uses lower case letters for all ASCII letters; however, case requirements do not apply to non-ASCII letters such as Greek, Cyrillic, or non-ASCII Latin letters.
Polyglot markup uses the correct case for element names.
Polyglot markup uses the correct case for attribute names.
definitionurl must be changed to the mixed case
Polyglot markup uses lowercase letters for the values of the attributes in the following list when they exist on HTML elements. More specifically, where required, polyglot markup must use lower case letters for all ASCII letters in these attribute values; however, case requirements do not apply to non-ASCII letters such as Greek, Cyrillic, or non-ASCII Latin letters. Attributes for HTML elements other than those in the following list may have values made of mixed case letters. All attributes on non-HTML elements may have values made of mixed case letters.
Polyglot markup uses only the elements in the following list as empty elements.
Polyglot markup uses the minimized tag syntax for empty
The alternative syntax
allowed by XML gives uncertain results in many existing user agents.
Given an empty instance of an element whose content model is not
EMPTY (for example, an empty title or paragraph) polyglot markup does
not use the minimized form (e.g. the document uses
Note that MathML and SVG elements may be either self-closing or contain content.
Polyglot markup does not contain line breaks and multiple white space characters within attribute values. These are handled inconsistently by user agents.
Polyglot markup surrounds all attribute values with quotation marks. Attribute values may be surrounded either by single quotation marks or by double quotation marks.
See also Attribute Values.
Polyglot markup uses only the following named entity references:
For entities beyond the previous list, a ployglot document uses
character references. For example, polyglot markup uses
Script and style commands should be included by linking to external files rather than including them in-line. However, polyglot markup must not link to an external stylesheet by using the xml-stylesheet processing instruction. See also Processing Instructions and the XML Declaration.
The following examples show the proper way to include external script and style, respectively:
<link rel="stylesheet" href="external.css"/>
are valid in an HTML document, neither function may be used in XHTML.
Therefore, neither is used in polyglot markup.
Instead, use the
innerHTML property for both HTML
Note that the
innerHTML property takes a string.
XML parsers parse the string as XML in XHTML.
HTML parsers parse the string as HTML in HTML.
Because of the difference in parsing, if you send the parser
content that does not follow the rules for polyglot markup the results
will differ for a DOM create with an XML parser and one created with an
Polyglot markup uses external scripts if that document's script
or style sheet uses
Note that XML parsers are permitted to silently remove the
contents of comments; therefore, the historical practice of hiding
scripts and style sheets within comments to make the documents backward
compatible is likely to not work as expected in XML-based user agents.
If polyglot markup must use script or style commands within its
source code, either use safe content or wrap the command in a CDATA
However, polyglot markup does not use a
section unless it is being used within foreign content.
Safe content is content that does not contain a
The following example is safe because it does not contain
problematic characters within the
Note that you cannot achieve same DOM in both XHTML and HTML by
using in-line commands in a CDATA section.
However, this is not usally a problem unless the code has a
dependency on the exact number of text nodes under a
The following examples show in-line script and style commands
wrapped in a
<script> //<![CDATA[ (script goes here) //]]> </script>
<style> /*<![CDATA[*/ (styles go here) /*]]>*/ </style>
When using MathML or SVG, the parser follows the XML parsing rules. Polyglot markup does not rely on getting a CDATA instance from the DOM when using MathML or SVG, because the HTML parser does not create a CDATA instance in the DOM.
Many thanks to Daniel Glazman, Tony Ross, Sam Ruby, Jonas Sicking, Henri Sivonen, and Philip Taylor. Special thanks to the W3C TAG.
No informative references.