HTML/CSS/JS has some huge advantages from the user perspective. When a designer has made some questionable choices, all it takes is a user stylesheet or a quick tour to the devtools to fix the font size and contrast ratio.
The whole world of extensions that improve existing websites with additional buttons etc. is only possible because the page structure is exposed in the DOM. If Reddit were only WASM rendering on a <canvas>, there were no "Better Reddit" extensions.
