Why it matters
Lazy content that pops in without reserved space shoves the text the reader is looking at.
How to fix it
Give lazy images/embeds dimensions or aspect-ratio and use min-height placeholders for late sections.
More smoothness rules
void smooth reports 13 rules in this category. How the page feels after it loads: dropped frames during a scripted scroll, long animation frames, INP of real clicks, shifts after input, animated layout properties, idle rAF loops and reduced-motion handling.
long-frames-during-scrollLong animation frames block scrollingscroll-listener-nonpassiveNon-passive wheel/touch listenerinp-slowInteractions are slow to respond (INP)layout-shift-after-inputLayout shifts long after an interactionanimate-layout-propertyAnimation of a layout-triggering propertytransition-alltransition: allwill-change-overusewill-change is applied too broadlyreduced-motion-ignoredAnimations keep running with prefers-reduced-motion