Not OP, but: You can definitely do encryption in the browser, no problem.
What you can’t do is make sure the server giving you the code that does the encryption doesn’t change it out with code that also sends your secret messages to Siberia. Unless you control the server giving you the web page. And you trust DNS.
At that point your trust model is simpler with an auditable local application. It could even be an Electron app - so almost exactly doing E2EE in the ‘browser’.
What you can’t do is make sure the server giving you the code that does the encryption doesn’t change it out with code that also sends your secret messages to Siberia. Unless you control the server giving you the web page. And you trust DNS.
At that point your trust model is simpler with an auditable local application. It could even be an Electron app - so almost exactly doing E2EE in the ‘browser’.