Minecraft 1.21 is significantly more demanding than 1.8. Browsers have memory limits, and rendering the complex lighting and entity data of modern Minecraft in JavaScript is a heavy task.
As of now, as a standalone, stable client. Beware of websites claiming to host it, as they are often just re-skinned 1.8 versions or, worse, phishing sites. eaglercraft 1.21
Some users use cloud computing to stream a full 1.21 Java instance to their browser. The Verdict Minecraft 1
Some servers use plugins to allow 1.8 clients to join 1.21-compatible worlds. You won't see the new blocks, but you can interact with the community. as a standalone