Works on both desktop & mobile, but it was designed for mobile users.
It's client-side only.
It captures the current frame of the stream and saves it to a file. That means you won't get the raw quality like when you play on a console, but it's still better than using the built-in screenshot feature on your phone.
Screenshot's resolution & quality depend on the quality of the stream at the moment.
Screenshot doesn't include touch UI, notification bar... only the gameplay.
There might be a slight delay.
⚠️ It's not possible to map the Share/Screenshot button on your controller to this feature.
How to
Enable this feature in the Settings.
Play a game.
Tap once at the bottom left/right (depending on your setting) to show the Screenshot button.
Tap on that button to capture screenshot.
Screenshot will be saved by the browser.
You can double-tap that corner to capture screenshot.