Bed

Every embed gets a bed.

An embed arrives in a page as somebody else's markup and behaves like it. It loads whatever it wants whenever it wants, and it settles to a height nobody knew in advance — after the reader has already started reading, so the paragraph they were halfway through jumps down the screen.

Bed lays a bed for each one. Space is reserved from heights measured on real page views, loading is deferred until the embed is wanted, and connections are warmed only for the embeds that are wanted immediately.

Free. Craft 5.3+, PHP 8.2+. No outbound requests, no third-party services, no build step.

Reference point: the WordPress Embed Optimizer plugin, and the Optimization Detective plugin it leans on for the measurement half. Bed does both halves itself, because a Craft site should not have to install two plugins to stop a tweet from moving the page.