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Riot-develop
The team developing the Riot app for Matrix regularly make a beta version with new features, improvements, and fixes available as a web app1 at https://riot.im/develop
Just login there with your usual Matrix username and password. If you have a tzchat.org account (or any non-matrix.org account) you should use your full Matrix username2 as username, something like @myusername:tzchat.org.
Once logged in, open up user settings (click your name at upper left and then choose Settings), click the Labs tab, and select beta features to enable by moving their control slider to green status.