For all who really want to know about HTML, here is the standard: http://www.w3.org/TR/REC-html40/.
And the standard for cascading style sheets CSS: http://www.w3.org/TR/PR-CSS2/.

Then there is a nice list which shows which html targets are supported by which version of Netscape and Internet Explorer (up to NS4.7 and IE5.0) and in which version of the standard they are defined: http://www.ncdesign.org/html/list.htm

And some tips how not to do it: HTML Hell

MudGuard