The overlap nobody mentions
Contrast. Text that fails a contrast check for someone with low vision is the same text that vanishes for everyone else outdoors. Every visitor is temporarily low-vision on a train platform at noon.
Target size. Buttons big enough for someone with limited motor control are buttons big enough for a thumb on a moving bus. Nobody has ever complained that a button was too easy to press.
Real labels on form fields. Screen readers need them; so does anyone who tapped into field four and cannot remember what it wanted, because the placeholder disappeared the moment they started typing.
Focus order that follows the page. Keyboard users depend on it, and it is also the thing that decides whether tabbing through a checkout feels like a form or like a maze.
None of this is a separate workstream. It is the same set of decisions, made with one more person in mind.
Where design usually breaks it
Editorial layouts are where it gets hard. My Hair PL arrived with its art direction already made: a hard fifty-fifty split, model on one side, product still life on the other, and a serif headline breaking across the seam so the two halves read as one page.
That composition is the brand. It cannot be simplified into a stack of centred boxes for narrow screens without losing the thing the client paid for, and it also cannot ship as an image with text baked into it.
The answer is the unglamorous one: rebuild the relationship rather than the arrangement. Which part leads when there is no room for both, what the type does when the seam disappears, and where the single red button goes when everything else has moved.
Five checks worth doing first
Tab through your most important page without touching the mouse. If you cannot reach the primary button, or cannot see where you are, that is the finding.
Run the contrast on your body text and your buttons, not the headline, which is usually fine, but the small grey text everyone forgets.
Turn images off and read the page. What is missing is what your alt text should have said.
Zoom to 200%. Text should reflow, not clip.
Check that every form field has a label that stays visible while typing.