>> At some point in mid-2017, all new CSS will be built with Quantum plane parts, not Gecko. Going forward, it will be much easier and more enjoyable to implement new CSS properties. Which makes it easier for folks in the open source community to contribute.
If you're not careful this can drag on for years with half the stuff done one way and half the new way - especially once you reach the point where "all new stuff is built the new way". There can come a point where you need to just purge the old stuff. We've seen this in places like GIMP moving to GEGL, and several project moving to GTK3 (GIMP, Inkscape, and others), the purge of java from LibreOffice, and now there's this mushy migration to Wayland. Firefox is not the only project migrating to rust either.
In all these cases I'm actually in favor of the migrations, and many of those have been completed over years. It's just that once a path has been defined I feel like there needs to be a more concerted effort to actually complete the transition sooner rather than later. Let new features take a back seat and get your internals switched over. I know it's a balance but working on a project that's straddling the fence is almost always slower in the long run.
Having said that, firefox is huge so subsystems will need to switch to rust one at a time. I'm just saying that once CSS can be done the new way they need to require it to all be done that way, not just new stuff. Perhaps that is the case and it wasn't spelled out clearly in TFA.