When does the page rendering process begin?

The page rendering process begins when the browser receives and starts processing the HTML document. This process includes several stages:

1. HTML Parsing: Building the DOM (Document Object Model) tree.
2. Building the CSSOM (CSS Object Model): Parsing the CSS applied to the page.
3. Building the Render Tree: Combining the DOM and CSSOM to create a render tree containing visible elements on the page and their styles.
4. Layout (Reflow): Determining the geometric sizes and positions of each element on the page.
5. Painting: Rendering the pixels of the elements on the screen.
6. Compositing: Combining the painted layers into the final view on the screen.

Although the rendering process starts on the client side (browser), its initiation is triggered by receiving the HTML, which is often dynamically generated on a Node.js server. The server is responsible for preparing and sending the initial HTML response.