This is not a flash-game knockoff — it's a full osu!standard engine running in your browser tab: stable-accurate hit windows, proper slider physics, spinners, HD/HR/DT/EZ/HT mods, skinning via .osk files and replay playback. Drop in any .osz beatmap and play it exactly like in the client.

Because the urge to play doesn't check whose PC you're on. School laptop, work break, a friend's machine, Linux box without Wine setup — open the page, throw in a map, play. It's also the fastest way to show osu! to someone: no 5GB install before their first click. Performance-wise a mid-range laptop holds a stable framerate; the engine is tuned for low input latency in Chromium-based browsers.
The web player is a funnel into the real thing. When browser plays stop being enough: register (you'll want the account for web scores anyway), then plug the client into osudesu and start climbing the PP leaderboard — including Relax and Autopilot ladders you won't find on the official server.
Yes — free like the rest of osudesu. No trials, no locked modes.
You can try the gameplay without one; an account is needed to submit scores to the web leaderboard and keep replays.
It runs, and replays are perfectly watchable on mobile; for competitive play you'll want a mouse or tablet on a desktop browser.
Chromium-based browsers (Chrome, Edge, Opera) give the best latency; Firefox works too.
Yes — export it as .osk and load it in the player settings.
To the web player's own leaderboard and your profile's web tab. Client plays feed the main PP ladders.