I'm also interested in Servo as well. With React Native, we had the liberty to restrict certain paradigms that don't map well to building a parallelizable pipeline of operations. So the JS thread can't synchronously query for layout, which happens in the next stage, and the layout stage can't synchronously block on reading main thread (UIKit etc) values.
By restricting what the programmer can do, it naturally formed a pipeline where layout can be performed in the second stage while JS is determining the next UI update and so on. I'm not sure the extent to which this benefits us right now, but even mobile devices are beginning to have four cores so it seems like a nice property to maintain as long as it doesn't cause problems.
Servo sounds cool, and I'm curious how it takes advantage of a similar architecture but while the programmer has free reign to "clog the pipeline" by synchronous queries on layout etc. I'm sure they've thought of it, I'd just like to hear what they came up with.