How does the browser render pages? What is compositing?

The page rendering process in a browser can be broken down into several stages:

1. Parsing HTML — the browser reads HTML and constructs the DOM tree.
2. Parsing CSS — the CSSOM (CSS Object Model) is built.
3. Building the Render Tree — combining DOM and CSSOM into a render tree containing visible elements with their styles.
4. Layout (Reflow) — calculating the sizes and positions of elements on the page.
5. Painting — rendering pixels on layers (layers can be created for optimization).
6. Compositing — merging all layers into the final image displayed on the screen.

Compositing is the process of combining multiple layers into a single final image. Modern browsers optimize performance by rendering some elements on separate layers (for example, using CSS properties like transform, opacity, will-change, etc.). This allows updating only the changed layers without re-rendering the entire page.

Thus, compositing helps improve performance and smoothness of animations by minimizing the number of re-rendering and re-calculating layout operations.

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For example, if you have an opacity animation on an element, the browser can create a separate layer for that element and, when the opacity changes, just recompose that layer without touching the rest of the page.