I don't know exact what system you have in mind, but writing a performant shader is hard. Requiring that designers attach arbitrary shader code to HTML elements is an easy way to absolutely tank the performance of the web.
Flexbox also isn't great at all for many, many use cases - its performance absolutely tanks outside of the use case it was designed for, specifically a 1D flow of blocks along an axis. If you want a grid layout, choose the grid layout algorithm.
Any system here must accommodate extremely heterogeneous requirements, so it will inevitably become "bloat". One alternative future you could envision is based on WASM and WebGPU, where each site is essentially an app that pulls in whatever libraries and frameworks it needs to do its work, but that's also pretty far off, since there is not sufficient standardization of the protocols used by WASM UI frameworks.
I wouldn't expect devs to be writing their own shaders all the time - the browser could have standard shaders, and no doubt libraries would crop up that offer more.
Flexbox also isn't great at all for many, many use cases - its performance absolutely tanks outside of the use case it was designed for, specifically a 1D flow of blocks along an axis. If you want a grid layout, choose the grid layout algorithm.
Any system here must accommodate extremely heterogeneous requirements, so it will inevitably become "bloat". One alternative future you could envision is based on WASM and WebGPU, where each site is essentially an app that pulls in whatever libraries and frameworks it needs to do its work, but that's also pretty far off, since there is not sufficient standardization of the protocols used by WASM UI frameworks.