When an element moves while someone is reading or clicking, the person may lose their place or select the wrong control. Unsized images and ads, dynamically inserted content, and font changes can cause shifts.
Specify image and video dimensions or aspect ratios and reserve space for ads and embeds. Choose a font-loading strategy after checking fallback font metrics and the rendered result.
Lab tests and real-user data reflect different conditions. Check for shifts during scrolling and interaction as well as initial loading. Reporting a specific improvement requires actual measurements taken under comparable conditions.