It means that if you accidentally break something in your draft, your browser (if compliant; most are, except Internet Explorer) will tell you:
- that you really did break something, and
- what that thing is.
This means you can fix it straight away in the draft, which will allow you to publish with confidence once your draft is error free.
Compare the alternative. If you serve your xhtml as text/html, then if your draft is broken, you’re not guaranteed to notice it at all. If so, you could well publish invalid markup without realising it.
Why’s that a problem, though? It’s a problem because as long as that invalid code is hanging around, it might affect all sorts of things. For instance, it might trigger ‘quirks mode’ in a browser other than the one you checked your draft with, causing people who use that browser to view your page to see it rendered differently than designed. Or it might interact with a later update to the page; the likelihood being, in this case, that the poor soul making the update might think it’s their code that’s caused the problem, leading them to waste time fruitlessly trying to debug it.
That’s why it’s cool 