In plain language
The reading order in the code must match the order that makes sense visually.
Who it affects
Screen-reader and keyboard users who encounter content in DOM order.
How to check
Read the page top-to-bottom with a screen reader (or disable CSS): the content must still make sense in that order. Worked example — A calendar app's event-creation form: disable CSS on a multi-step form: check whether instructions appear before their related fields in the DOM — instructions coded after the fields they govern arrive too late for screen reader users.
Common misconception
CSS can rearrange what people see without changing what's in the code. A sighted user and a screen-reader user can end up experiencing two completely different reading orders from the same page. Real example: on a dating app's profile setup, content is visually re-arranged with CSS so the order you see on screen is different from the order a screen reader reads it — the meaning changes or gets confusing when read aloud.