Most of the big ones are things like skia, harfbuzz, wgpu - all totally reasonable IMO.
The two that stand out for me as more notable are html5ever for parsing HTML and taffy for handling CSS grids and flexbox - that's vendored with an explanation of some minor changes here: https://github.com/wilsonzlin/fastrender/blob/19bf1036105d4e...
Taffy a solid library choice, but it's probably the most robust ammunition for anyone who wants to argue that this shouldn't count as a "from scratch" rendering engine.
I don't think it detracts much if at all from FastRender as an example of what an army of coding agents can help a single engineer achieve in a few weeks of work.
Also graphics acceleration makes it hard to do from scratch rather than using using the 3D APIs but I guess you could in principle go bare iron on hardware that has published specs such as AMD, or just do software only rendering.