Shaping is different compared to rendering glyphs themselves. SDF renderers (and other GPU text renderers like Slug) still do shaping on the CPU, not in shaders. Maybe some experiments have been done in this area, but I doubt anyone shapes text directly in the GPU in practice.

Think of it like a function that takes text as input, and returns positions as output. Shaders don't really know anything about text. Sure you could probably implement it if you wanted to, but why would you? I think it would add complexity for no benefit (not even performance).

1) Because SDFs suck badly (and don't cover the whole field) when you want to render sharp text. SDFs are fine when used in a game where everything is mapped to textures and is in motion at weird angles. SDFs are not fine in a static document which is rendered precisely in 2D.

2) Because GPUs handle "conditional" anything like crap. GPUs can apply a zillion computations as long as those computations apply to everything. The moment you want some of those computations to only apply to these things GPUs fall over in a heap. Every "if" statement wipes out half your throughput.

3) Because "text rendering" is multiple problems all smashed together. Text rendering is vector graphics--taking outlines and rendering them to a pixmap. Text rendering is shaping--taking text and a font and generating outlines. Text rendering is interactive--taking text and putting a selection or caret on it. None of these things parallelize well except maybe vector rendering.

No part of the rasterizer or renderer is configurable here. As mentioned above, the rasterizer is already programmable with up to two different bespoke stack bytecode languages, but that has nothing to do with shaping through wasm.