Accessibility — site usable by everyone. Not ethics — it's law in EU/US.
Blind users
Screen reader reads aloud. Need alt tags, semantic HTML.
Color blind
Don't convey info only by color. Contrast 4.5:1+.
Keyboard
Everything works without mouse. Tab + Enter.
Semantic HTML
<button>, <nav>, <main> instead of generic <div>.
WCAG 2.1 AA
Most laws require.