...a rough approximation, and your mileage may vary.

Specifically, preview doesn't filter messages, it just displays exactly what you type as HTML. IE is forgiving and lets you type malformed HTML, but not all browsers are so loose, and I expect all browsers to get stricter as time goes on. The filter running on the server is strict about enforcing correct HTML, mainly because it has to: It looks for the quotes to mark the beginning and end of a value so that it can understand it. I didn't want to burden it with a ton of extra code to look for all the myriad ways that humans can screw up HTML.