Why wouldn't you do this conversion on the server side, where you have tighter control of the XSLT parser? Why leave anything to chance? Personally, when I've done XML/XSLT -> HTML presentation conversion, I've always done it on the server. YMMV. I guess there's something to be said for offloading conversion to the client, but I've always worried about the very thing you are, compliance.
That said, most browsers you'll encounter today (safari, webkit, gecko, IE) will have decent XSLT support. Just like HTML it'll have it's quirks, however. IIRC with XSLT, like XHTML, your HTML needs to be well-formed. No
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