receiver will understand your data correctly. It's semantics vs. syntax. You can easily write a perfectly valid HTML4-Strict compliant document and have it render differently in different browsers. Why? Because they all implement the CSS differently. Web browser interoperability problems actually have nothing to do with HTML parsing; it's all about the rendering, which is totally unrelated. It's amazing to me how many people think strict HTML validation will actually solve any real-world problems. Unsurprisingly, my earlier comments about XML and Postel's Law caused a bit of a flamewar in the various places that have flamewars about these things. What did surprise me, though, is that people have mostly heard of Postel's Law... they just think it's wrong. That's actually not what I expected; I naively assumed that people were doing things they way they do because they simply didn't know any better. Oh well, live and learn. As for me, I'm a huge Postel fan, and I have been for years. For example, a little over five years ago, I wrote my own replacement for the curses (text-mode display handling) library for the same reason that I recently wrote my own permissive (albeit incomplete) XML parser: because nobody else was following Postel's Law. You can still read my original article about it . Probably people will flame me now for being stupid enough to write my own replacement for curses, just like I was stupid enough to write my own XML parser; however, the fact remains that my terminal