https://tabnav.com/academy/wcag/success-criterion-2.1.1
Understanding WCAG SC 2.1.1: Keyboard | tabnav Academy
WCAG Success Criterion 2.1.1 explained with tips to ensure all functionality is accessible by keyboard.
understanding wcagsckeyboardacademy
https://www.w3.org/WAI/GL/2014/WD-UNDERSTANDING-WCAG20-20140107/complete-diff
Understanding WCAG 2.0 -- Diff-marked Version
understanding wcagdiffmarkedversion