(I don't think they've swapped out the font renderer and image codecs yet, but there are quite a few bits and pieces they have swapped out as the Rust community delivers more pure Rust libraries.)
Too good, in fact: it won't run on any laptops that I own because their GPUs don't support a sufficient level of OpenGL.
One great aspect of Firefox-with-Gecko is that you can throw it onto just about any machine with more than 512MB of RAM and it will provide bootstrap web access ( slow or otherwise ). That's going to be lost when everything is Servoised. I guess it's back to links2 at that point.
EDIT: to be clear, with the Servo nightlies you need to do this yourself but Firefox could bundle it eventually.
For ease of development, webrender2 is configured to compile with the latest OpenGl 4.x features. I believe there is a way to build it to target a lower OpenGl version.
Edit: Correction, its currently targeted at OpenGl 3.2. There's an open issue for bringing it down to OpenGl 3.1 which is the version supported by integrated Sandybridge GPUs. Considering 40% of people running Intel are using Sandybridge or older, that's probably why it wont run on your laptops.
Intel Corporation Mobile GM965/GL960 Integrated Graphics Controller
OpenGL renderer string: Mesa DRI Intel(R) 965GM
Maximum OpenGL version for that chipset is 2.0, apparently, though I can't find a way to push it beyond 1.4 on Linux
I appreciate that six years old is ancient by SV standards but I would be interested to learn what proportion of Firefox users are on equally old hardware. I did try launching Servo with CPU rendering but it hung indefinitely.
I don't think the software fallback has gotten much love but AFAIK it'll work fine once llvmpipe support is added to webrender2 [1]. Once that is done, rendering will work with the vast majority of laptops and desktops running Linux/MacOS/Windows. The OpenGL version supported by that chipset is over a decade old so devoting time to compatibility this early in the project would be a waste of time (and defeat the purpose since the standard predates the explosion of mobile devices).