Sadly it's not a native API, but a clever hack that effectively comments-out source of the page with a legacy `<plaintext>` element (proving that old cruft is really hard to kill in HTML :)
Some of my coworkers made a firefox os app installation page that had both <marquee> and <blink>. At first I thought they were doing some crazy css animation thing to get scrolling, flashing text on the phone. Nope, just marquee and blink.
Capturing as a native feature of the browser is something that we really hope to see down the line. There are many reasons you might want to disable the pre-parser, and our goal is to empower the developer to make that decision themselves. I'd default it ON though :)