Surface — lay the phone flat on the thing you're levelling. Centre the bubble in the small ring.

Edge — stand the phone on its edge against a shelf or frame, portrait or landscape. The bar reads the tilt.

Zero — set the current angle as level if your phone's case or camera bump tilts it.

Motion sensors only work on a phone or tablet, over https or as a local file. Inside a published page they're blocked — use the demo there.