WCAG, published by the W3C, sets testable success criteria organized around four principles. Content must be perceivable, operable, understandable, and robust. Version 2.2 is current, and level AA is the usual target for businesses and public bodies.
Practical accessibility includes sufficient color contrast, keyboard access to everything, meaningful link text, text alternatives for images, labeled form fields, and support for screen readers. Building it into components from the start costs far less than retrofitting.