Oww, that is interesting use of servo. Regarding the parsing I may point to work of @alcinnz
For wasm I made some survey some time ago: https://discuss.coding.social/t/solidground-wasm-component-based-development-wiki/367
I'm interested in server-side use mostly, but some impressive experiment is Makepad, that offers a Figma-like DSL for complex UI. In their conference vid multiple apps are live-embedded in their presentation slidedeck.