But that doesn't seem like something that should be standardized anyway. There is no one way to do web UI, and the current popular paradigm (DOM/CSS) isn't particularly good either. Going to a lower-level gives more control to web developers, and the browser could always cache popular frameworks.

Now that we think about it, even with the DOM/CSS UI framework built into the browser, people still make use of libraries like jquery so library caching is something the browser needs to do anyway, WASM or not.