Tell about the critical stages of browser rendering.

Critical stages of browser rendering include:

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 to determine what and how to display.
4. Layout (Reflow) — calculating the exact positions and sizes of elements.
5. Painting — rendering pixels on the screen.

Any changes in the DOM or styles can trigger a re-layout or repaint, affecting performance.