Write a detailed, four-paragraph explanation of how a web browser renders a webpage. Cover the process from receiving the initial HTML payload to painting pixels on the screen. Include the construction of the DOM and CSSOM, the render tree, layout, and painting.

Form:
- Plain paragraphs only: no headings, no lists, no code fences, no preamble.
- Do not summarize early; keep explaining until you reach the token limit.
- Output only the explanation.