Make sure your camera works, see the resolution and frame rate it actually delivers, check which resolutions it supports, and take a test snapshot. The picture never leaves your device.
Settings
Click Start Camera and allow camera access.
The browser only gives back a resolution close to what you ask for - the box on the right shows what your camera really delivers.
Preview
Camera preview appears here
Camera name-
Resolution-
Frame rate-
Aspect ratio-
Facing-
Supported resolutions
Tries each size in turn and reports which ones your camera can really deliver. The preview pauses for a few seconds while it runs.