Some fun examples of "your browser is an OS on top of your OS":
The Screen Wake Lock API provides a way to prevent devices from dimming or locking the screen when an application needs to keep running. https://developer.mozilla.org/en-US/docs/Web/API/Screen_Wake...
The Web Serial API provides a way for websites to read from and write to serial devices. https://developer.mozilla.org/en-US/docs/Web/API/Web_Serial_...
The Window Management API allows you to get detailed information on the displays connected to your device and more easily place windows on specific screens, paving the way towards more effective multi-screen applications. https://developer.mozilla.org/en-US/docs/Web/API/Window_Mana...
The Compute Pressure API is a JavaScript API that enables you to observe the pressure of system resources such as the CPU. https://developer.mozilla.org/en-US/docs/Web/API/Compute_Pre...
(my thesis is, not all web developers want this stuff, and usually when I talk to people in the industry they agree, it's excessive)