The WHATWG Blog

Please leave your sense of logic at the door, thanks!

Charmod Norm Checking

Wednesday, November 15th, 2006

Charmod Norm is still in the Working Draft state, but if it were to become a normative part of (X)HTML5, it would belong to the area of the conformance checking service that I am working on now, so I prototyped Charmod Norm enforcement as well.

The checker outsources most work to ICU4J. Most complexity in my code is due to trying to avoid buffering as much as possible while still using the ICU4J API unmodified—and due to dealing with the halves of a surrogate pair falling into different UTF-16 code unit buffers. On the spec reading front, I couldn’t map “the second character in the canonical decomposition mapping of some character that is not listed in the Composition Exclusion Table defined in [UTR #15]” to the ICU4J API on my own. Fortunately, I got excellent help on the icu-support mailing list.

It turned out that the most time-consuming part was not writing the normalization checker but reworking how Ælfred2 deals with US-ASCII, ISO-8859-1, UTF-8, UTF-16 and UTF-32. In HS Ælfred2, all character encodings are now decoded using the java.nio.charset framework.

Requirements

The definition for Fully-normalized Text involves checking normalization before and after parsing. That is, the source text is required to be NFC-normalized and after parsing the constructs parsed out of the source are required to be NFC-normalized and are required not to start with a “composing character” (which is not exactly the same as a “combining character”).

I don’t really like the way the definition involves peeking underneath the parser, but it does have the benefit that if the source is in NFC, you won’t accidentally break the document by editing the source in an NFC-normalizing text editor.

Interpretation

Charmod Norm does not define what “constructs” are in the context of XML 1.0 or HTML5.

However, XML 1.1 does define what “relevant constructs” are, so that definition might be generalizable to XML 1.0 and HTML5. Unfortunately, XML 1.1 defines relevant constructs in terms of the grammar productions of XML itself instead of the significant information items that an XML processor reports to the application.

Personally, I think the XML 1.1 definition is neither practically useful nor something for which I’d be motivated to write an implementation. So for the purpose of prototyping, I made up a definition on my own. Web Applications 1.0 just might get away with making my definition normative for XHTML5 considering that XML 1.0 doesn’t have a definition.

I consider the SAX2 ContentHandler (excluding qNames) and DTDHandler benchmarks of cluefulness when it comes to XML-related spec, API and application design. In general, if your application isn’t an editor that needs to reconstruct the XML source from parsed data, your application is most likely broken if it needs to know something about an XML document being parsed that is not exposed through those two interfaces. On the other hand, a spec that can’t be conformed to by viewing XML through only those two interfaces is broken. Moreover, DTDHandler is about notations, which are pretty much obsolete, so that leaves only ContentHandler.

This gives the following definition of constructs:

Implementation

There is a new pseudo-schema called http://hsivonen.iki.fi/checkers/nfc/. It is enabled for all (X)HTML presets. When this pseudo-schema is in use or when the schema selection is in the automatic mode, the normalization checking of source text underneath the parser is enabled as well.

The following checks are made:

The version of Unicode that is used is 5.0.0.

The column and line numbers reported on errors are very inaccurate due to buffering.

I have not tested whether all the character encoding decoders that I have installed are normalizing transcoders. If you have Windows-1258 Vietnamese test cases, please try them out and let me know what happens. Also, please let me know if the issue applies to something other than legacy Vietnamese encodings.

As usual, the new code is enabled for testing. Please let me know if it doesn’t work as described.

Posted in Conformance Checking, Syntax | 1 Comment »

HTML vs. XHTML

Wednesday, November 15th, 2006

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 DOCTYPE.

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 text/html, it gets parsed as HTML; but if it is served with an XML MIME type, like application/xml or application/xhtml+xml, it gets parsed as XHTML.

Document Serializations

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. <ol>, <ul>, etc.) 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:

  1. The browser that currently holds the majority market share doesn’t support XHTML (that is actually served and processed as XHTML).
  2. 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.
  3. 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.

The New DOCTYPE

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: 

<!DOCTYPE html>

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 DOCTYPE.

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.

Conformance Checking

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 (X)HTML5.

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.

Posted in Conformance Checking, Syntax | 6 Comments »