Editing Polyglot (computing) (section)

===Polyglot markup===

'''Polyglot markup''' has been proposed as a useful combination of the benefits of [[HTML5]] and [[XHTML]].<ref name="w3c">{{cite web |title=Polyglot Markup: A robust profile of the HTML5 vocabulary |url=https://www.w3.org/TR/html-polyglot/ |access-date=4 September 2022 |archive-date=9 August 2022 |archive-url=https://web.archive.org/web/20220809042911/https://www.w3.org/TR/html-polyglot/ |url-status=live }}</ref> Such documents can be parsed as either HTML (which is <span style="white-space:nowrap">[[SGML]]-compatible</span>) or [[XML]], and will produce the same [[Document Object Model|DOM]] structure either way. For example, in order for an [[HTML5]] document to meet these criteria, the two requirements are that it must have an HTML5 [[Document Type Declaration|doctype]], and be written in well-formed XHTML. The same document can then be served as either HTML or XHTML, depending on browser support and MIME type.

As expressed by the ''html-polyglot recommendation'',<ref name="w3c" /> to write a polyglot HTML5 document, the following key points should be observed:

# Processing instructions and the XML declaration are both forbidden in polyglot markup
# Specifying a document’s character encoding
# The DOCTYPE
# Namespaces
# Element syntax (i.e. End tags are not optional. Use self-closing tags for void elements.)
# Element content
# Text (i.e. pre and textarea should not start with newline character)
# Attributes (i.e. Values must be quoted)
# Named entity references (i.e. Only amp, lt, gt, apos, quot)
# Comments (i.e. Use &lt;!-- syntax --&gt;)
# Scripting and styling polyglot markup

The most basic possible polyglot markup document would therefore look like this:<ref name="w3c" />
<syntaxhighlight lang="html">
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
  <head>
    <title>The title element must not be empty.</title>
  </head>
  <body>
  </body>
</html>
</syntaxhighlight>

In a polyglot markup document non-void elements (such as <code>script</code>, <code>p</code>, <code>div</code>) cannot be self-closing even if they are empty, as this is not valid HTML.<ref>[http://dev.w3.org/html5/html-xhtml-author-guide/html-xhtml-authoring-guide.html#empty-elements Polyglot Markup: HTML-Compatible XHTML Documents: 6.4 Void Elements] {{Webarchive|url=https://web.archive.org/web/20121002041508/http://dev.w3.org/html5/html-xhtml-author-guide/html-xhtml-authoring-guide.html#empty-elements |date=2 October 2012 }}. W3C Editor's Draft 9 July 2012.</ref> For example, to add an empty textarea to a page, one cannot use <code><nowiki><textarea/></nowiki></code>, but has to use <code><nowiki><textarea></textarea></nowiki></code> instead.