Is HTML and CSS parsing done sequentially or in parallel?

Parsing of HTML and CSS in the browser occurs in a specific order, but not entirely sequentially or in parallel.

First, the browser loads the HTML and begins constructing the DOM tree. When it encounters a <link> tag for CSS or a <style> tag, it loads and parses the CSS, building the CSSOM (CSS Object Model).

HTML parsing is paused if external CSS is encountered to avoid rendering without styles (FOUC). CSS is parsed in parallel with loading, but the application of styles occurs after the CSSOM is built.

Thus, loading and parsing CSS happen in parallel with HTML loading, but the application of styles and further rendering wait for the CSSOM to be completed. This ensures the correct display of the page.