I updated WTP to version 0.7.1 today and something curious happened: the option to edit HTML files using the HTML editor disappeared. Strange, I thought. When I reverted back to WTP 0.7, the ability returned.
I created a completely new and separate installation of Eclipse 3.1.1 to verify the problem. After loading WTP 0.7.1, everything worked as expected (I could open the HTML editor). However, after I loaded PHPEclipse 1.1.7, the ability to open the HTML editor disappeared again. I’ve raised bug #1336929 against PHPEclipse to address the problem.
In the meantime, I can still edit HTML files using the PHPEditor which seems–so far–to work fine. Using the PHPEditor has the added benefit of automagically updating the PHP Browser view (to preview the page) whenever I save.