Vector graphics are really hard to do on a GPU in an efficient manner. The way the data is stored as individual curve segments makes it difficult to parallelize the coverage problem, it's equivalent to a global parse; the best approaches all do some form of parsing of curve data on the CPU, either rasterizing fully on the GPU, or putting it in a structure the GPU can chew on.
But again, this has nothing to do with HarfBuzz or wasm.
But again, this has nothing to do with HarfBuzz or wasm.