#Conversejs (.org) is web client for #xmpp with #omemo available (click green pad)
If you want to use omemo it has to be available on BOTH clients
You may try movim.eu, but I'm not sure if it supports omemo on its web/browser interface. Their server has it and works fine with android client.
jami.net is worth you consider it, NO web client, but it has clients for all platforms, really decentralized and FSF endorsed