HTML Validator FTW Rated 5 out of 5 stars

HTML Validator is the easiest way to find validation errors and fix them according to the schemas. (Now, the schemas for xhtml are broken in various places such as css and javascript inside xhtml, but that is not HTML Vallidator's fault.)
FEATURE REQUEST FOR HTML VALIDATOR: html5
FEATURE REQUEST FOR FIREFOX: one year with no API changes

This review is for a previous version of the add-on (0.9.0.6).