> "rasterise the viewport and use it as a tile"

Yes, rendering HTML/CSS using solely Unity's backend is problematic at best.

For the Unreal Engine though it is perfectly feasible.

Here is Sciter Engine (HTML/CSS/script) rendered directly into DirectX scene: https://sciter.com/see-sciter-lite-unreal-engine-in-theaters... using Sciter's DX rendering backend.