Should I write Polyglot HTML5 documents?

Question

I've been considering converting my current HTML5 documents to polyglot HTML5 ones. I figure that even if they only ever get served as text/html, the extra checks of writing it XML would help to keep my coding habits tidy and valid.

Is there anything particularly thrilling in the HTML5-only space that would make this an unwise choice?

Secondly, the specs are a bit hazy on how to validate a polyglot document. I assume the basics are:

1. No errors when run through the W3C Validator as HTML5
2. No errors when run through an XML parser

But are there any other rules I'm missing?

Thirdly, seeing as it is a polyglot, does anyone know any caveats to serving it as application/xhtml+xml to supporting browsers and text/html to non-supporting ones?

Edit: After a small bit of experimenting I found that entities like   break in XHTML5 (no DTD). That XML parser is a bit of a double-edged sword, I guess I've answered my third question already.

Answer 1

Work on defining how to create HTML5 polyglot documents is currently on-going, but see http://dev.w3.org/html5/html-xhtml-author-guide/html-xhtml-authoring-guide.html for an early draft. It's certainly possible to do, but it does require a good deal of coding discipline, and you will need to decide whether it's worth the effort. Although I create HTML4.01/XHTML1.0 polyglot documents, I create them using an XML tool chain which guarantees XML well-formedness and have specialized code to ensure compatibility with HTML non-void elements and valid XML characters. Direct hand coding would be very difficult.

One known current issue in HTML5 is the srcdoc attribute on the iframe element. Because the value of the attribute contains markup, certain characters need to be escaped. The HTML5 draft spec describes how to do this for the HTML serialization, but not (the last time I looked) how to do it in the XHTML serialization.

Answer 2

I'm late to the party but after 5 years the question is still relevant. On one hand closing all my tags strongly appeals to me. For people reading it, for easier editing, for Great Justice. OTOH, looking at the gory details of the polyglot spec — http://www.sitepoint.com/have-you-considered-polyglot-markup/ has a convenient summary at the end — it's clear to me I can't get it all right by hand.

https://developer.mozilla.org/en/docs/Writing_JavaScript_for_XHTML also sheds interesting light on why XHTML failed: the very choice to use XML mime type has various side effects at run time. By now it should be routine for good JS code to handle these (e.g. always lowercase tag names before comparing) but I don't want all that. There are enough cross-browser issues to test for as-is, thank you.

So I think there is a useful middle way:

1. For now serve only as text/html. Stop worrying that it will actually parse as exactly the same DOM with same runtime behavior in both HTML and XML modes.

2. Only strive that it parses as some well-formed XML. It helps readers, it helps editors, it lets me use XML parser on my own documents.

   Unfortunately, polyglot tools are rare to non-existant — it's hard to even serialize back XML in a way that also passes the HTML requirements...

   • No brainer: always self close void tags (<hr/>) and separately close non-void tags (<script ...></script>).

   • No brainers: use lowercase tags and attr (except some SVG but foreign content uses XML rules anyway), always quote attribute values, always provide attribute values (selected="selected" is more verbose than stanalone selected but I can live with that).

   • Inline <script> and <style> are most annoying. I can't use & or < inside without breaking XML parsing. I need:

     <script>/*<![CDATA[*/
        foo < bar && bar < baz;
     /*]]>*/</script>
     

   ...and that's about it! Not caring about XML namespaces or matching HTML's implied DOM for tables drops about half the rules :-)

3. Await some future when I can directly go to authoring XHTML, skipping polyglotness. The benefits are I'll be able to forget the tag-closing limitations, will be able to directly consume and produce it with XML tools. Sure, neglecting xml namespaces and other things now will make the switch harder, but I think I'll create more new documents in this future than convert existing ones.

   Actually I'm not entirely sure what's stopping me from living in that future right now. Is it only IE 8? I'm also a tiny bit concerned about the all-or-nothing error handling. I'm slighly hoping a future HTML spec will find a way to shrink the HTML vs XML gaps, e.g. make browsers accept <hr></hr> and <script .../> in HTML— while still retaining HTML error handling.

   Also, tools. Having libraries in many languages that can serialize to polyglot markup would make it feasible for programs to generate it. Having tools to validate and convert HTML5 <-> polyglot <-> XHTML5 would help. Otherwise, it's pretty much doomed.