Can a website read the battery of an Xbox controller?

Not through the standard Gamepad API. Use the Xbox Accessories app on Windows or the battery indicator in the Xbox Game Bar.

Why does the battery jump from 100 to 90 percent?

The controller itself reports battery in steps, not single percent values, so every tool that reads it sees the same jumps.

Yes. The page sends the same temporary output report a game would. It does not touch firmware or stored settings.

Connect a PlayStation controller with WebHID to read its reported battery level and charge state, and to light up the LEDs directly.

Not connected

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The controller reports its charge in steps of about 10 %. The value shown is the middle of that step, the same convention the Linux driver uses.

Light bar color

Player indicator (DualSense)

Rumble motorsStrong (left)0%Weak (right)0%

Changes are temporary output commands, the same kind a game sends. Nothing is stored on the controller.

Use Chrome or Edge on a desktop computer.
Connect a DualSense, DualSense Edge or DualShock 4 by USB or Bluetooth.
Press Connect and choose the controller in the browser prompt.
Read the battery panel, then try the light bar, player LEDs and rumble controls.

Battery level: PlayStation controllers report their charge in coarse steps of roughly ten percent, so the number moves in jumps. Treat it like the console battery icon, not a lab measurement.
Charging state: Shows whether the controller says it is discharging, charging or full. A controller on a cable that never reports charging may have a worn port or a cable without power lines.
Light bar and LEDs: Colors you set here are temporary. They reset when the controller reconnects or a game takes over, and nothing is saved to the controller.

Can a website read the battery of an Xbox controller?: Not through the standard Gamepad API. Use the Xbox Accessories app on Windows or the battery indicator in the Xbox Game Bar.
Why does the battery jump from 100 to 90 percent?: The controller itself reports battery in steps, not single percent values, so every tool that reads it sees the same jumps.
Is it safe to change the light bar from a website?: Yes. The page sends the same temporary output report a game would. It does not touch firmware or stored settings.