First Paint (FP)
FP marks the point when the browser renders anything that is visually different from what was on the screen prior to navigation.
First Contentful Paint (FCP)
FCP is the point when the browser renders the first bit of content from the DOM, which may be text, an image, SVG, or even a canvas element. See Page (Load|Speed) Metric - First Contentful Paint (FCP)
- puppeteer has a argument –show-paint-rects that visibly render a border around paint rects in the web page to help debug and study painting behavior.