Some of the new structural elements (header, footer, nav, article, section) should help simplify CSS based web design and make it easier to move a stylesheet from one design to another without having to remap classes or ids. The new attributes on the input tag (date, date-time, range, email, etc.) will be nice so that we don’t have to rely on javascript to do things that are common enough that the browser could handle them (should save us a few bytes too).

See all of the changes in HTML 5.