I like to be alerted of any validation errors that sneak into my markup as soon as possible. That's why I find the HTML Validator Extension for Firefox very hard to live (well, work) without.
Unfortunately, it does not support HTML5. I've been looking hard for an alternative that does and has the same features as the HTML Validator Extension, i.e. runs locally in the browser without sending anything to a remote server and validates every page you load automatically.
No luck.
Posted in Browsers, HTML 5, Web Standards.