Web Content Accessibility Guidelines
There's the WCAG. It is made by the W3 Consortium. It is THE guideline for accessible web content and baseline for most accessibility laws and policies.
See: https://www.w3.org/WAI/standards-guidelines/wcag/
Having a glance at this can be overwhelming. It still is very overwhelming for me.
For accessibility conformance testing, they've also put together a guideline. It starts with a couple easy checks followed by more testing in detail.