There's a lot of refactoring to do in Gecko's graphics stack to fit the new rendering model. While they do that, they can also accelerate one thing at a time with WebRender, testing incrementally. Things like SVG which WebRender doesn't have yet, can also use the old system and be included in WebRender as images.

The old system will still be needed post-WebRender for printing at the very least.

> The old system will still be needed post-WebRender for printing at the very least.

Isn't it possible to take a frame from WebRender and print that? Having two separate systems seems like allot of unnecessary maintenance. Especially if one of those systems is only used on rare occasions like printing.

My understanding is that WebRender doesn't support everything yet, so right now it's mostly about validating correctness.

Ah I see, so it basically enables a parallel run of webrender to gather compatibility/correctness stats.

So it renders it two times and compares the results?

It renders it in series -- stuff goes through webrender first, and then whatever webrender couldn't handle gets dealt with with the regular renderer.