Six Units, Three Inputs, One String
ISO 8601 durations look simple until you try to edit them. PT1H30M is fine as a string — but asking a user to type PT1H30M into a text input is asking for syntax errors. The property palette’s default resolver needed a proper control for format: "duration" schemas, and the parent spec (#373) explicitly deferred it because a multi-field compound UI is a different beast from the single-native-input controls like date and number.
The interesting design choice was the field set. ISO 8601 durations span six units — years, months, days, hours, minutes, seconds — but years and months are calendar-ambiguous (a month is 28–31 days depending on context). I defaulted to hours/minutes/seconds with a fields array property for consumers who need more. The array determines which sub-inputs render and in what order, and hidden units are silently dropped on round-trip. If you set value="P1YT2H" but only show ['hours'], the year vanishes when the value serializes back — the control edits only what it shows.
The implementation reused the “plain <input> to avoid side-effect registration” pattern from pages-slider. Each sub-input is a raw <input type="number" min="0"> with a short unit label below — h, m, s. Not a <pages-number-input> custom element, because importing the slider shouldn’t force-register pages-number-input as a side effect. The same reasoning applies here.
Serialization omits zero-valued units — PT1H30M not PT1H30M0S — with the canonical empty duration being PT0S. The regex parser handles the full ISO 8601 duration grammar but only populates the fields the consumer asked for.
The resolver change was a single line: format: "duration" maps to pages-duration-input, alongside the existing date and date-time mappings. The palette’s barrel import picks up the side-effect registration. 19 tests cover parsing, serialization, field configuration, ARIA, and readonly/disabled states.
What this opens up: the palette now handles every editor type from the original #373 spec. The remaining follow-ups are #375 (migrating PagesSchemaForm to embed the palette) and the blocks-ui migration (which needs the palette published to .casehub-packages first).