A Lizard that Does HyperText HTML support has been woeful in most browsers up to now. My article, HTML 4.0 in Netscape and Explorer , tried to catalog the differences between the specification and the two major implementations. As expected, it's a long piece, and it even skips one or two areas as too complicated.
Gecko is far beyond that point in many respects.
First of all, it supports a lot more of HTML 4.0. And second, it doesn't choke on proper HTML 4.0. It's difficult to catalog all of Gecko's capabilities since little documentation exists about it, and short of browsing the source there's little to tell you what is supported and what isn't. But this isn't the point. The point is to show some of the progress that has been made thus far.