Gemini for pages and if you really need an app, wasm and webgl/webgpu. HTML/CSS/JS can definitely be replaced by better, already existing things, but browsers will still need to understand those for many years going forward, even if everyone suddenly stopped developing with them.
Webapps don't really have semantics. Accessibility is the toolkit's job, leaving that to the web platform invariably generates half baked results in an app.
Gemini for pages and if you really need an app, wasm and webgl/webgpu. HTML/CSS/JS can definitely be replaced by better, already existing things, but browsers will still need to understand those for many years going forward, even if everyone suddenly stopped developing with them.