I am a designer whose main marketing strategy is multi browser compatibility. I assure my clients that the site will work even in IE6 (!).
Of late i have been pondering
As a rule of thumb I would not develop a site purely in HTML 5 but would consider using it for certain, richer, parts of the site. Remember that it is still not recommended by W3C and IE barely supports it at all.
This blog has a good discussion on it: http://blogs.forrester.com/ronald_rogowski/10-05-10-what_should_customer_experience_professionals_do_about_html5