The answer
It blocks HTML parsing and rendering. The browser stops parsing, downloads the script, executes it, then resumes. This makes the page feel slow, especially for large scripts.
How async and defer Impact Page Rendering
How does a normal script tag affect page rendering?
More FAQs in How async and defer Impact Page Rendering
Only briefly. async downloads in parallel (no blocking). But when the script is ready, it pauses parsing to execute. The pause is brief but can cause a janky render.
No. defer downloads in parallel and executes only after all HTML is parsed. The page renders fully before deferred scripts run. This is the most non-blocking option.
After all deferred scripts execute. DOMContentLoaded waits for defer scripts. This means deferred scripts are guaranteed to run before DOMContentLoaded.
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