The 3D API is just meant to be a minimal portable abstraction over common low-level hardware features, composing those features into higher level constructs is intentionally left as an exercise for the user.
Web APIs need more standardized functionality built in, including high-level ones, and not rely on additional libraries, because they have to download across a network.
It's like having to install an app every time you visit a site.
https://github.com/linebender/vello
and (shameless plug):
The last time I tried with pixi.js the problem was smoothly zooming polygons with a constant width border thicker than hairline. Doing that was basically just generating new textures on every frame.