Firefox actually has partial validation built-in: view source and see if something is red.
This validation is future-compatible, meaning that it won't find all errors (such as unknown tags or attributes).
This validation is future-compatible, meaning that it won't find all errors (such as unknown tags or attributes).
I'm pointing this out because of a recent blog post by Mozilla dev Henri Sivonen which says: "Please do not advertise the new View Source implementation by saying that Firefox now has a validator in the View Source window."
(see full post here for all the details, there's a section named Limitations: http://hsivonen.iki.fi/view-source/)