SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/tech-script-blocks-parsing

While building a page, what makes the browser pause parsing the HTML?

A classic script without async or defer.

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  1. A classic script without async or defer ✔
  2. An image tag
  3. A CSS color value

Why: The browser turns HTML into the DOM and CSS into the CSSOM. A classic external script without async or defer pauses HTML parsing while the script is fetched and executed.

Huh, didn't know that: The critical rendering path has five steps: build the DOM, build the CSSOM, make the render tree, run layout, then paint.

The dad joke
My web page loaded slowly. The script would not let anyone else talk until it finished.
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References

Answer: A classic script without async or defer. The browser turns HTML into the DOM and CSS into the CSSOM. A classic external script without async or defer pauses HTML parsing while the script is fetched and executed.

  1. MDN: How browsers work developer.mozilla.org/en-US/docs/Web/Performance/Guides/How_browsers_work
    block rendering, and pause the parsing of HTML.
    Checked 2026-10-11.

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