Since browser vendors have traditionally not adhered to one standard, and also due to the (theoretical!) possibility of buggy browsers, I think this comes down to empirical testing, not standards. If you write a web app, you have to test it on the browsers you expect "most" of your target users to be using. I think this will remain true when (or if?) the main browsers become XHTML-based, since users will potentially continue to use all kind of (old) browsers just as at present.