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[w3.org ]
The Firefox web developer extension has validation buttons AND a lot of other useful stuff.
I would suggest testing in multiple browsers as you go. It sounds too late for you do that in this case.
I have taken to working mostly in Firefox (because of the web developer extension and for standards compliance) and testing in Opera, Konqueror and IE at regular intervals and whenever I make major changes.
The commonest problem is things breaking on IE that work on everything else, but there are bugs in the other browsers as well.
The issue is to learn how each browser renders the same things, determine how close they actually have to be, and code your pages accordingly.