And for those who don't follow Rust closely, it's important that it's "the DOM thread only," as Rust's opt-in GC is on a per-task (thread) basis, so the threads that don't use GC don't even know it's there or running.
(Note that Rust doesn't actually have its own GC yet, Servo is just using spidermonkey's JS GC. pcwalton's comment implies it is being used single-threadly though.)
There can be multiple script tasks even within a single page. For example, cross-domain or sandboxed iframes will have each DOM and script in its own task. Also, each of those will have their own layout and rendering tasks.
