Expose "paintTime" and "presentationTime" in element timing, LCP, long animation frames, and paint timing. "paintTime" means the time when the rendering phase ended and the browser started the paint phase. "presentationTime" means the time when the "pixels reached the screen", which is somewhat implementation-defined. This feature entry omits event timing, which would be done separately.
So far the spec defined the paint time as the time after the "rendering update", when the document hands over rendering to the UA. However, in chromium the exposed paint time (in event timing, element timing, LCP and paint-timing) was different - the approximated VSync time from the compositor, which is important in terms of UX. This created confusion and incompatibility This proposal defines both these timestamps, and exposes them in an identical way in all the relevant entries.
Explainers: https://github.com/w3c/paint-timing/blob/main/presentation-timestamps.md