Explain the difference between layout, painting and compositing.

Layout, painting, and compositing are stages in the browser's rendering process to convert your HTML, CSS, and JavaScript into a pixelated view on the user's screen. Let's break down each one:

1. Layout (or Reflow): The browser calculates the size and position of each visible element on the page to build the render tree. This process involves taking into account the viewport size, element dimensions, positioning, and other styles. If you've ever used CSS Flexbox or Grid, the browser is doing a lot of work during the layout stage to figure out how those items fit together based on their relationships and available space. Any changes to an element's dimensions, the addition or removal of elements, or changes to an element's content can trigger a reflow.

2. Painting (or Rasterizing): After layout is complete and the positions of elements are known, the browser starts painting. Painting is the process of filling in pixels. It involves drawing out text, colors, images, borders, and shadows, essentially every visual part of the elements. The painting stage is where your site starts to look like something recognizable to the user. Changes to an element's color, background, or visible content can trigger a repaint.

3. Compositing: This is the final phase of the rendering process. Here, the browser takes all the painted parts of your page and layers them together in the correct order to give the final page. Compositing is necessary because you might have overlapping elements, and the browser needs to know which elements appear over others. In addition, some elements might need to be on their own compositor layers to take advantage of hardware acceleration. Changes that involve an element's position or its visibility can trigger compositing work.

In terms of performance, it's worth noting that layout is generally the most costly of these processes. Layout changes can often trigger subsequent paint and composite operations, leading to a significant performance impact. On the other hand, composite-only changes are the least costly and can often be handled by the GPU, leading to smoother animations and transitions. This is why developers aiming for 60 frames per second animations aim for composite-only changes.