Largely spurred by our ramp up of fancy JavaScript.
My pitch to make this a front burner issue is based on the idea that, when faced with bad html, browser parsers will "keep on chugging" and make guesses about tag termination. Different browsers make different guesses. This results in differnt DOM structures for the same page. Different DOM structures freaks out the JavaScript on dynamically updated (AJAX style) pages and renders them unusable. We now spend 5-10 times the effort to clear QA because of JS errors caused by bad HTML. So to lower costs and improve agility, we must start validating our html using [link|http://validator.w3.org|http://validator.w3.org]