Welcome back to "This Week in HTML 5," where I'll try to summarize the major activity in the ongoing standards process in the WHATWG and W3C HTML Working Group. The primary editor was traveling this week, so there are very few spec changes to discuss. Instead, I'd like to try something a little different.
It has been suggested (1, 2, 3, &c.) that HTML 5 is trying to bite off more than it can metaphorically chew. It is true that it is a large specification, and it might benefit from being split into several pieces. But it is not true that it includes everything but the kitchen sink.
For example, HTML 5 will not
Daniel Schattenkirchner asked whether Almost-Standards mode is still needed. Almost-Standards mode is a form of DOCTYPE sniffing invented by Mozilla in 2002 to address line heights in table cells containing images. Bug 153032 implemented the new mode, which Mozilla called "Almost Standards mode" and HTML 5 calls "limited quirks mode." Henri Sivonen made the point that it would probably be too expensive to get rid of the mode. Like it or not, we're probably stuck with it.
And finally, a gem that I missed when it was first discussed: back in July, "Lars" provided the best documentation of the
<keygen> element, ever.
Tune in next week for another exciting episode of "This Week in HTML 5."
One of the predominate issues raised in the recent call for comments was surrounding
the whole HTML vs. XHTML debate, with many people suggesting that we should
not be extending HTML, but rather focussing on XHTML only.
However, what many people have failed to realise is that HTML5 resolves this issue: The (X)HTML5 specification
is in fact specifying extensions to both HTML and XHTML simultaneously, and
the choice of using either is no longer dependent upon the
Many authors use an XHTML 1.0
DOCTYPE and then proceed
to claim they’re using XHTML, but browsers make the decision whether to
treat a document as HTML or XHTML based on the MIME type. (X)HTML5 endorses
dispatching on MIME type: If the document is served as
it gets parsed as HTML; but if it is served with an XML MIME type, like
it gets parsed as XHTML.
HTML5 introduces the concept of serialisations for an HTML document. A serialisation
in this context refers to the physical representation of the essence of the
document—the document tree. (X)HTML5 requires user agents that support scripting
to expose the document tree to the script using the DOM API and data model.
HTML5 uses the HTML serialisation and XHTML5 uses the XML serialisation.
Because of this, the distinction between an HTML and XHTML document is reduced.
In most cases, either serialisation can be used to represent exactly
the same document. The main differences are that the HTML serialisation, due to
backwards-compatibility reasons cannot represent structured inline-level elements (e.g.
as children of the
<p> element and the XML serialization cannot
represent all possible document trees that may be created as a result of error
recovery in the HTML parsing algorithm. Also, in browsers, some scripting API features and
CSS layout details work differently depending on the serialisation of the document due to
backwards compatibility considerations.
The XML serialisation used by XHTML must be a well-formed XML 1.0 document.
However, unlike previous versions of HTML, the HTML serialisation is no longer
considered an application of SGML, but instead defines
its own syntax. While
the syntax is inspired by SGML, it is being defined in a way that more closely
resembles the way browsers actually handle HTML in the real world, particularly
in regards to error handling.
The HTML5 serialisation and the accompanying parsing algorithm are needed for three reasons:
- The browser that currently holds the majority market share doesn’t support XHTML (that is actually served and processed as XHTML).
- The legacy
text/html content out there needs a well-defined parsing algorithm—something that SGML-based HTML specifications haven’t been able to provide.
- There are content management systems, Web applications and workflows that are not based on XML tools and cannot produce well-formed output reliably. These systems can benefit from new features even though they wouldn’t work reliably with the XHTML serialisation.
On the other hand, thanks to the HTML/XHTML duality, new systems can be built on solid off-the-shelf XML tools internally and convert to and from the HTML5 serialisation at input/output boundaries. Once the installed base of browsers supports
application/xhtml+xml properly, these systems can swap the output serialiser and start using XHTML-only features such as lists inside paragraphs.
In practice, the
DOCTYPE serves two purposes: DTD based validation
and (for HTML only)
DOCTYPE sniffing. Since HTML is no longer considered
an application of SGML and because there are many limitations with DTD based
validation, there will not be any official DTDs for (X)HTML5.
As a result, in the HTML serialisation, the only purpose for even having a
DOCTYPE is to trigger standards mode in browsers. Thus, because it doesn’t
need to refer to a DTD at all, the
DOCTYPE is simply this:
I’m sure you would agree that that is about as simple and easy to remember
as possible. But, for XHTML, it’s even simpler. There isn’t one! Since
browsers have not (and will not) introduce
DOCTYPE sniffing for
XML, there is little need for a
However, I should point
out that there is one other minor practical issue with DTDlessness in
XML. Entity references which are declared in the XHTML 1.0 DTD will
not be able to be used. However, since browsers don't use validating parsers
and do not read DTDs from the network anyway, the use of entity references
is not recommended. Instead, it is recommended to use character references
or a good character encoding (UTF-8) that supports the characters natively.
You’re no doubt wondering, if there are no DTDs, how will one go about validating
their markup. Well, that’s simple. There are in fact other, more robust methods
available for checking document conformance. There are several different schema
languages that can be used, including RELAX NG and Schematron. However, even
they cannot fully express the machine-checkable conformance requirements of
Henri Sivonen is in the process of developing a conformance
checker for HTML5,
which is being designed to report much more useful error messages beyond
those that are possible using just a DTD based approach. For example, the table
integrity checker discussed previously is one feature that is impossible
to implement using DTDs.