"Properly"? I'm not convinced. Browsers have grown incredibly large and complex. Every plugin it replaced by JS-based APIs has become a security nightmare in almost exactly the same way as the plugin based approach. Browsers have a lot of features, but if they were implemented "properly", the user would have far more control than they do now. The cynic in me thinks that many of these features are mostly there to facilitate ad-tech and user tracking. From fingerprintable canvas to access to devices.
Applets gave you opaque binaries with full system access by default; the web at least gives you mechanisms to audit, limit, and turn off capabilities.
And yes, a lot of the modern API surface exists because ad-tech pushed the envelope — no disagreement there. But the same standardization that helped them also enabled PWAs, WASM, privacy-preserving modes, and tools like TeaVM.
So maybe not “properly” as in “perfect,” but “properly” as in “the ecosystem finally has levers for users and developers that plugins never offered.”
TeaVM and similar toolchains show that the original idea behind applets wasn’t wrong — the implementation model was. Moving Java to JS/WASM with tree-shaking, minification, and real browser APIs gives you all the benefits without the security nightmare.
The interesting takeaway isn’t nostalgia for applets, but how mature the web stack has become: the browser is finally the runtime applets always wanted.