Checking for stick drift
If your character or camera moves on its own after you let go of a stick, it may be drifting. With your hands off, the dot should sit inside the dashed center circle (a value of 0.1) and X and Y should be close to 0. Most games apply a dead zone that ignores small values, so a tiny offset may never matter in play.
Circularity
Push the stick to the edge and rotate it slowly a few times to trace its outer range. Average error is how far that outline strays from a circle of radius 1, averaged over all angles. Many controllers read above or below 1 on the diagonals, so 0% is rare. Use it to compare the two sticks on one controller, or the same stick over time.
Button names and mapping
Browsers translate common controllers into a standard layout. With standard mapping, button 0 is the bottom button of the right-hand cluster (A on Xbox, ✕ on PlayStation, B on Nintendo). If a controller is shown as non-standard, the picture may not match its real layout, so read the numbers in Raw values instead. The vibration test appears only when both the browser and the controller support it.