Playbook: Decide collapse-vs-reserve per slot

Playbook: Decide collapse-vs-reserve per slot
What it does: the empty-slot dilemma — collapse for UX or reserve for zero CLS. Here's the rule.

Setup (effort 2/5):
— ATF and anchor slots: ALWAYS reserve, never collapse. Their shift is the costliest for CWV.
— Deep in-content slots with high no-fill: collapse on empty to avoid big gaps — they shift less.
— Use collapseEmptyDivs(true) only AFTER the bid resolves, not before render — premature collapse then expand IS a double shift.
— Better than collapsing: reserve the slot's median creative height so filled and empty match.
— Measure CLS before/after the change; the right call differs per slot position.

RPM lift seen: indirect via CWV protection on the slots that matter.

Pros: ends a common one-size mistake. Cons: per-slot logic adds config.

Lift: ★★★☆☆ — Risk: ★☆☆☆☆
Best for: sites with variable fill. Skip if every slot fills ~100%.