I have seen people put Flash on a page that checks the domain environment variable. If it's not running on the correct domain, the Flash terminates. The Flash movie contains code that triggers Javascript events, writes Javascript that in turn writes HTML onto the page, and controls many of the actions of the page. Since Flash can be compiled and obfuscated, it's easier for the thief to write their own page than to copy the content.
It's not a trivial process but for someone who wants to protect their HTML (which I don't understamd), it is somewhat possible.
Of course, protecting PHP is easy because it's parsed server side and never reaches the client.