Ask HN: Go HTML5 only for a new product? 16 years ago
First of all, the HTML5 and CSS3 standards are not finished yet. It means semantic values of tags, attributes, and even tag names, as well as their functionalities, can change at any time, and other can appear. Thus, it seems difficult to build an HTML5-only website for the general public.
Moreover, if this website is designed for a large company, there are chances that its user base is still using IE (even IE6 maybe). In this case, HTML5 and CSS3 are not welcome, even for IE8.
I would recommend to wait for IE9 release and the standards to be finished. Maybe you can add extra content for people using modern web browsers, like the <video> tag, but I think IE's user base is too large to be ignored.