paint timing API
The Paint Timing API keeps detailed information on two events – the first paint and the first ‘contentful’ paint, which refers to something more than just styling, such as text or an image.
The Performanceobserver interface describes what to do when these events occur. This can then be used to send the data to a server for analysis later on.
Right now, this is only available in Chrome and Opera, but other browsers such as Edge and Safari have both shown an interest and will possibly include it in the future. • w3.org/tr/paint-timing