The Web Content Accessibility Guidelines (WCAG) 2.2 are the W3C standard for accessible websites. Level AA is the level most laws and companies ask for. It covers things like readable contrast, a site you can use with only a keyboard, motion you can stop, and a structure screen readers can follow.
There is also a level AAA. The W3C itself does not recommend requiring it for a whole site, because some content can’t meet it. I went for AA everywhere and adopted a few AAA practices where they cost little and help a lot: larger touch targets, a very visible keyboard focus, and animation that switches off when you ask your device for less motion.
I chose not to add an accessibility widget (the floating panel with font size and contrast buttons). Your browser and operating system already do that better, and those widgets often conflict with screen readers. The site should be accessible by default, not after you press a button.