Progressive disclosure

Reveal information as it is needed rather than all at once. Costs an interaction; buys a large reduction in cognitive load. Use it where most users need only part of the content.

Anatomy: visible summary → labelled control → disclosed content, adjacent and in flow.

Rules

• The control is descriptively labelled. "Benefits of a custom domain", not "Read more". It must make sense read aloud out of context, because screen readers announce links and buttons in isolation.
• Disclosed content appears next to its trigger, not elsewhere on the page. Content that opens where the user is not looking has not been disclosed.
• State is visible: the control shows whether the content is open or closed.
• Use <details>/<summary> unless you need behaviour they cannot express (H-48). Keyboard, screen-reader and no-JS support come free.
• Conditional fields are opt-in, not optional. A "Mobile number" field revealed by ticking "Receive updates by text" is simpler than a permanently visible optional field — people who do not want it never see it, and people who do get a required field with an obvious reason.
• Never hide with it what E-63 requires visible. Progressive disclosure is for depth, not for labels, selected states or primary actions.

Mode variance: operator discloses least. For daily users, one dense visible view beats a tidy one that hides what they came for.