Demaestro - 8:49 pm on May 31, 2010 (gmt 0)

Does anyone know of any reasons that creating new pages in HTML 5 would be bad if other pages on the site are HTML4.1?

Just curious what side effects mixed doc type sites may incur.

if a tag does not affect the rendering, then why is it there? I can't think of a single tag like this I've used.

They don't change the rendering but they still add value in the markup. Usually it is to give you more control over container tags. A good example of this is <tbody> and <theader>