Neither broke web pages, honestly. XHTML requires a DTD named at the top of the document, and browsers will happily fall back to HTML 3, 4, or 5 as they can if there’s no DTD specified.
My interpretation of "break web pages" was serving XHTML with MIME type application/xhtml+xml, in which case browsers don't render anything when the XHTML isn't well-formed, which is really just a strict / validate-my-syntax mode you can opt into.
