These are my oft-answered responses to Frequently Asked Questions in the various Usenet newsgroups that I read. It seems simpler to refer a questioner here, rather than looking up stuff to repost over and over.
Transitioning? What DOCTYPE are you using in your web document? Is it a Transitional or Strict doctype? Transitional is supposed to be used when you are updating an old legacy page, and you don't have time to move all the old presentational HTML in the document into CSS. [ <p font-size: 2> ... ] and last-century things like that. New documents should be HTML 4.01 Strict.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">