What happens during HTML parsing by a browser? How are CSS and JavaScript files processed?

When parsing HTML, the browser sequentially reads the HTML document and constructs the DOM tree — a structure reflecting the page elements.

Alongside this, the browser detects links to CSS and JavaScript files:

• CSS is loaded asynchronously, after which the CSSOM (CSS Object Model) is built. Then, the DOM and CSSOM are combined into the render tree, which is used for rendering the page.

• JavaScript usually blocks HTML parsing unless the script is marked as async or defer. When a <script> tag is encountered, the browser stops parsing, loads, and executes the script, as it may modify the DOM or CSSOM. After executing the script, parsing continues.

Thus, the process looks like this:

1. HTML parsing → building DOM
2. Parallel loading of CSS → building CSSOM
3. Combining DOM and CSSOM → render tree
4. Executing JavaScript (blocking or asynchronous)
5. Rendering the page based on the render tree

This ensures correct display and interactivity of the web page.