As mentioned earlier, there is an ongoing project for replacing Gecko’s old HTML parser with an HTML5 parser. Significant improvements have landed lately, so if you’ve previously tried the HTML5 parser and turned it off due to crashiness or Web compatibility issues, now is a good time to turn it back on.

What’s New?

• The correctness and crashiness fallout from the speculative parsing / off-the-main-thread parsing move has been fixed.
• The core parts of the HTML5 parser have been fixed to pass the applicable tokenizer and tree builder tests from html5lib test suite (except white space handling when reconstructing active formatting elements).
• <!--…--> escapes in <script> are parsed in a new magic way that is believed to be Web-compatible but does not require backtracking/reparsing. This is the most significant change. If you are a Web author and are wondering how to make the escapes robust on pages you write, the easiest approach is never to use the <!--…--> escapes in <script> and to escape script end tags within string literals as "<\/script>".
• <title>, <textarea>, <style> and <xmp> no longer allow the end tag to be masked with <!--.
• <table><form> now works in a Web-compatible way.
• Malformed <ul> and <ol> lists now work in a Web-compatible way.
• Starting with tomorrow’s nightlies, <frameset> parsing after junk markup works in a Web-compatible way.
• Numeric character refernces to astral characters now work correctly.
• Known (non-shutdown) memory leaks have been fixed.
• Huge text nodes (such as Tinderbox logs) are now flushed incrementally.

The changes to the parser core have also been deployed to Validator.nu.

How to Enable the HTML5 Parser?

First, this isn’t release-quality software. Testing the HTML5 parser carries all the same risks as testing a nightly build in general, and then some. It may crash, it may corrupt your Firefox profile, etc. If you aren’t comfortable with taking the risks associated with running nighly builds, you shouldn’t participate.

If you are still comfortable with testing, download a trunk nightly build, run it with a profile you are only using with trunk builds, navigate to about:config and flip the preference named html5.enable to true. This makes Gecko use the HTML5 parser when loading pages into the content area and when setting innerHTML. The HTML5 parser is not used for HTML embedded in feeds, Netscape bookmark import, View Source, etc., yet. Note that turning the pref on in a profile that you will also be using with Firefox 3.6 is most likely a bad idea, since on the 3.6 branch the pref enables obsolete code.

The html5.enable preference doesn’t require a restart to take effect. It takes effect the next time you load a page.

What to Test?

The main thing is getting the HTML5 parser exposed to a wide range of real Web content that people browse. This may turn up crashes or compatibility problems.

So the way to help is to use nightly builds with the HTML5 parser for browsing as usual. If you see no difference, things are going well! If you see a page misbehaving—or, worse, crashing—with the HTML5 parser turned on but not with it turned off, please report the problem.

Reporting Bugs

Please file bugs in the “Core” product under “HTML: Parser” component with “[HTML5] ” at the start of the summary.

Known Problems

First and foremost, please refer to the list of known bugs.