Great add-on Rated 4 out of 5 stars
Excellent at detecting errors (in fact, FAR better than "html validator").
Another plus point is that you get to see a visual display of your pages (with numbered lines) and a comprehensive reason behind each error that might occur.
On the downside, it sends the html file to it's own external database (where it is stored for a period of time ?how long????) rather than perform the check "on site" as it where (locally on your computer).
Apart from this, it's a great html validator to use.
Web pages that are validated are normally deleted as soon as the validation is completed. Screenshots are normally held between 30 mins and an hour to allow the results to be served by a web server