Polyglot Az

This specification summarizes design guidelines for authors who wish their XHTML or HTML documents to be conforming whether parsed as HTML or as XML. The document is intended to be useful to web authors, in particular those who want to serve receivers without concern for whether they have XML or HTML parsers available. Such concerns may, for instance, arise in content syndication or when receivers are on legacy systems. HTML polyglots facilitate migration to and from XHTML, including transition from XML 1.x to HTML5, and this document serves to accurately specify the requirements of a UTF-8 based profile for such documents.

No recommendation is made in this document or by the W3C regarding whether or not to publish polyglot content. In general, authors are encouraged to publish HTML content using HTML5 syntax and media types (either HTML syntax and text/html, or XHTML syntax and application/xhtml+xml).

Polyglot Az

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It is sometimes valuable to be able to serve HTML5 documents that are also well formed 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. 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 that are both HTML5 documents and XML documents. It is recommended that these documents be served as either text/html (if the content is transmitted to an HTML-aware user agent) or application/xhtml+xml (if the content is transmitted to an XHTML-aware user agent). Other permissible MIME types are text/xml, application/xml, and any MIME type whose subtype ends with the four characters "+xml". [XML-MT]

Authors need not understand the benefits of robustness in order to benefit from the syntax of polyglot markup. However, in order to promote its benefits, it is necessary to understand that polyglot markup does not add semantics, and as such is not any more or less semantic than other flavors of HTML. Polyglot markup does, however, work to preserve semantics, including during the authoring process. Polyglot markup also does not ensure accessibility,as it does not add any accessibility requirements that other relevant specifications have not already added. But polyglot markup can work to preserve accessibility through adherence to required practices.

For the most part, polyglot markup is just a pure deduction of the validity constraints and syntax requirements that HTML and XHTML each dictate, many of which took "polyglotness" into consideration when they were added to HTML5. However, for reasons of robustness, this specification sometimes goes further than the principle of the lowest common denominator would have required.

Using robust syntax can enable documents to be parsed more reliable in less capable parsers. But even if the document can be expected to be parsed and validated by tools that fully conform to HTML5, polyglot markup adds robustness. As an example, when serialized as HTML, the closing tag for the p element is entirely optional and will be inferred if not present. But inclusion of closings tags, as required by XML and, thus, by polyglot markup, cause no harm beyond a minor increase in transfer size (an increase often mitigated by compression), but does allow validators to detect situations where the implicit closing rules don't match what the author intended.

Polyglot markup, itself being valid HTML5, supports extensibility as it is defined in Section 2.2.3 Extensibility of HTML5, so long as the extension does not violate the rules of polyglot markup. [HTML5] In addition, being well formed XML, polyglot markup can be extended when it is served as application/xhtml+xml.

[HTML5] introduces undeclared (native) support for attributes in the XLink namespace and with the prefix xlink:. To maintain XML-compatibility, polyglot markup explicitly declares the XLink namespace: xmlns:xlink=" "). [XML10]

The xml: namespace prefix used in xml:base, xml:lang, xml:space, and xml:id does not need to be declared in XML documents, and therefore polyglot markup does not declare these prefixes via xmlns. The prefixes are implicitly declared in XML and are automatically applied to the appropriate attributes in HTML. See CSS namespaces [CSS3NAMESPACE] how to use CSS selectors with these attributes.

Every polyglot markup document therefore contains an html, head, title, and body element. The html element is the root element. The head and body elements are children of the html element. The title element is a child of the head element. Therefore, the following is the most basic polyglot markup document.

Whenever it uses a tr element, polyglot markup always wraps the tr element inside a tbody, thead, or tfoot element. In HTML, if a group of one or more adjacent tr elements are not explictly wrapped inside a tbody, thead, or tfoot element, the HTML parser creates and wraps a new tbody element around the tr elements. XML parsers do not create the tbody element, thus offering the potential for creating different DOMs.

Whenever it uses col elements within a table element, polyglot markup explicitly uses a colgroup element surrounding groups of the col elements. In HTML, if a group of one or more adjacent col elements are not explicitly wrapped inside a colgroup element, the HTML parser creates and wraps a new colgroup element around the col elements. XML parsers do not create the colgroup element, thus offering the potential for creating different DOMs.

Polyglot markup uses lowercase letters for all HTML element names; all MathML element names; and all SVG element names except the following SVG element names, for which polyglot markup uses mixed case:

Polyglot markup uses lowercase letters in attribute names for all HTML elements; all MathML elements except the lowercase definitionurl, which polyglot markup changes to the mixed case definitionURL; and all SVG attributes except the following SVG attributes, for which polyglot markup uses mixed case:

Also note that because XML processors don't recognize lang as containing language information, polyglot markup uses both the lang and the xml:lang attributes (see Language attributes); however, the CSS3 Selectors specification stipulates that language attributes, including xml:lang, are matched in a case insensitive way. [SELECT]

In the HTML syntax, void elements are elements that always are empty and never have an end tag. All elements listed as void in the HTML specification or in an extension spec, MUST in polyglot markup have the syntactic form of an XML empty-element tag (). Other elements MUST NOT use the XML empty-element tag syntax. The void elements of the HTML specification are: [HTML5]

Example: 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 empty-element tag syntax. That is, the document uses and not .

Because an author may need to comment out the CDATA "start tag" and "end tag," polyglot markup allows for one node before and after the CDATA section. The following example has three nodes: one text node before the CDATA section, one for the CDATA section, itself, and one after the CDATA section:

Escapable raw text elements are elements in which character references are permitted but where the HTML parser treats elements as text rather than as markup. For polyglot markup, escapable raw text elements are:

When polyglot markup uses either a textarea or pre element, the text within the element should not begin with a newline. This is because HTML and SGML-based systems delete the initial newline on parsing, while XML parsers do not.

Within an attribute's value, polyglot markup represents tabs, line feeds, and carriage returns as numeric character references rather than by using literal characters. For example, within an attribute's value, polyglot markup uses 	 for a tab rather than the verbatim string literal, \t. This is because of attribute normalization in XML [XML10]. Note, too, that JavaScript and CSS in attribute values are affected by attribute value normalization, because a comment ends up commenting out not to the end of the source line but to the end of the entire attribute value.

Because of attribute-value normalization in XML [XML10], polyglot markup does not use newline characters within an attribute. Practically speaking, for source code with newlines within attributes, DOMs generated via XML and HTML will be different; however, whitespace differences have no behavioral impact on the page unless:

The following attributes are not allowed in HTML or XHTML within polyglot markup. These attributes have effects in documents parsed as XML but do not have effects in documents parsed as text/html. The HTML5 spec therefore defines them as invalid in text/html documents. [HTML5]

Note that the xml:space and xml:base attributes are allowed on SVG and MathML elements. The attributes may therefore appear in polyglot markup when they appear within SVG or MathML as foreign content.

When specifying the language mapping of an element, polyglot markup uses both the lang and the xml:lang attributes. Neither attribute is to be used without the other, and polyglot markup maintains identical values for both lang and xml:lang.

For entities beyond the previous list, polyglot markup uses character references. For example, polyglot markup uses   instead of  . Note that polyglot markup may use decimal values for escape characters (such as   in the previous example); however, the Character Model for the World Wide Web recommends that content SHOULD use the hexadecimal form of character escapes rather than the decimal form when both are available. [CHARMOD]

When applying JavaScript and CSS to polyglot markup, the goal is to get the same result whether consumed as HTML or as XML. It is therefore important to be aware of scripting and styling features that give different results in HTML vs XML. These issues comes in addition to the polyglot usage rules for raw text elements. 2351a5e196