Keyboard accessibility is one of the most important — and most neglected — aspects of web accessibility. An estimated 2.5 million Americans have motor disabilities that prevent mouse use. If your site can't be operated entirely by keyboard, you're excluding them completely.
The Four Core Principles
WCAG 2.2 Principle 2 (Operable) contains the keyboard requirements:
2.1.1 Keyboard (AA): All functionality must be operable via keyboard
2.1.2 No Keyboard Trap (AA): If focus moves into a component, it must be possible to move it out
