lucy24 - 4:28 am on Sep 4, 2011 (gmt 0)

Uhm, I hope you simply mean "correctly formatted" not "actually valid" because that takes you into a whole nother realm. You want something in the form blahblah at blahblah dot tld, and then something in the form {optional www.} blahblah dot tld, right?

Short answer: You cannot do it with html alone. But take heart: CSS won't do anything either; that's just for presentation.

Piece of cake in javascript, php or language of your choice. Betcha if you keep looking, you'll find a plugin that lets you constrain responses to a given format. Another common one is telephone numbers.

Oh, and if your generated html doesn't validate, find a different package.