I searched years for it: a webchat client for Jabber/XMPP, which is mostly secure and comfortable to use: Converse.js.
On our website in the bottom right corner there is now a "toggle chat"-button, when clicked existing users (@linuxlovers.at, @0nl1ne.at or @jabber.linuxlovers.at) can login:

And when you don't want this website but only the chat client, or you want to register an account, just visit jabber.linuxlovers.at - works even with our Captcha protected registration (available domains: @linuxlovers.at and @0nl1ne.at):