WebAssembly
A compact instruction format browsers run at near-native speed, so code written in other languages can work on a page without being rewritten in JavaScript.
It earns its place where the work is computation, not interface: image and video processing, compression, cryptography, simulation, and increasingly the runtimes that make a small model run inside a page instead of behind an API.
It is not a replacement for JavaScript and does not compete with it. It has no direct access to the DOM; JavaScript remains the layer that reads the page and hands work across, which is why an app that is mostly forms and clicks gains nothing from it.
Weight is the price. A module arrives as a separate download that must be fetched, compiled, and instantiated before the first result appears, so a feature used once in a session often costs more than it saves.