It sure does work for me, firefox on windows too.

Works on my Firefox on Windows, for shift + scroll focus need to be on the element you want to scroll. Arrow keys works even better

How do I get Shift here? (Safari on iPad)

1280 pixels. This is the width that should be enough for everyone according to github.

(max-width in .container-xl. Why did they put that there? The website looks perfectly fine without it)