TL DR: If you're a player, try this out and lemme know how it works with your system – which gamepad and which operating system (Windows / Mac) and browser.

If you're a game dev, I plan to make this open for everyone to use. Let me know if you'd like an implementation for your game engine.

Gamepad support in HTML5 is kinda sux because the JavaScript API behind it doesn't consistently map the same physical gamepad input to the same variables if players are using different gamepads and operating systems. So for my next game I made a scene where if the player has a gamepad they'll be prompted to press each of the buttons and move each of the sticks on the gamepad, and the game will read those inputs and map them to the in-game actions you want. I tested it out with my Switch controller on Windows and it works for all buttons except the left D-pad (which for some reason gives output to an Axis like with a stick instead of to Buttons) but that's fine for my game since the mapping works great for the left and right stick, the four right-side buttons, top and bottom left and right shoulder triggers, start, and select which is plenty.

Even better, I wrote code to save the gamepad configuration to LocalStorage so players will only ever need to configure their gamepad once. Even if they configure it in one game, they can still read that saved configuration data even from a different game, even if it's made in a different game engine. And I ultimately plan to make this public so any game devs on NG can use it in their games.

I'm saving it to LocalStorage as a JSON where the keys are button/axis names in human readable text (LStickDown, R2, ButtonUp [for the right pad's upper button] etc) and the values are the button number that the JavaScript API associates with it (and for axes it also has another key/value indicating whether it's the positive or negative direction). So it should be straightforward to import it into any game engine that can run JavaScript and handle return values, or get a JSON from LocalStorage through any other means. I've already written code for cross-game collabs in Unity, Godot, HaxeFlixel, and Phaser (JavaScript) that could be used to read the values in, so you would just need to handle them in a way that makes sense for your engine/framework.