These pages (should) comply with HTML 4.01 (Strict) and CSS2. The single exception is a non-standard property to persuade Microsoft Internet Explorer to do the correct thing and render PNG images with transparency correctly. Standard compliant browsers will render them in a reasonable fashion, and they should degrade reasonably on older browsers. The best (most compliant with standards) browser to use is probably one based on a recent version of the Gecko rendering engine: Mozilla, Galeon, Netscape 7. Recent versions of Microsoft Internet Explorer do OK (subject to the PNG caveat above.) Memo to Microsoft: doing the right thing when faced with <IMG SRC="image.png"> is not hard... Opera 7 is reputed to be reasonable. CSS support in Netscape 4 is basically broken; Netscape 4 users are advised to disable stylesheets or upgrade their browser.
You can use the links on the left of the page to check (with validators provided by the W3C) that these pages do comply with the relevant standards. Please tell me about any mistakes!