HTML 4.01 is the current standard.
HTML 5.0 is on the way (just google it), and many browsers are already making all of the necessary preparations to work with it.
It's also important to know that 'old' HTML code (code that worked in HTML 1.0, etc) may or may not work anymore. Most browsers now support the new stuff, but not necessarily the old. You can use something called a Doctype to declare, more specifically, what version (and type - frameset, strict, loose, transitional) of HTML code your document contains, so that the browser knows how to handle the code therein.