Validating HTML5 in the browser

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.

There are tools that validate markup by sending it to a remote server, either automatically or manually, but I don’t want that. I want to be able to check my work even though it’s behind a login or if I’m not connected to the Internet.

If you happen to know of a browser extension that solves this problem, please let me know. It doesn’t have to be for Firefox – Opera, Chrome or Safari would be fine too as long as it runs on my Mac.