I do a far share of Twilio development, and when I use their specialized markup Eclipse complains that it's not well formed (red squiggly line, not completing tag, etc).
Of course, as far as Eclipse knows, it's not - the files I'm editing are either .php or .phtml, so I don't fault it for assuming I should be using HTML tags.
How can Eclipse be configured to accept another set of tags in addition to HTML?