'requirements' kinda depends on who you are trying to sell to. If it's to the US Federal government, it needs to meet the criteria of what's known as 'Section 508'. There's information about that here:
https://section508.gov/
Other customers, such as schools and universities, might want WCAG 2.1
https://www.w3.org/TR/WCAG20/
They may also need to be 508 compliant because of funding rules.
Beyond that, things become complicated.