defer vs async on script tags

defer vs async on script tags
Both unblock the parser. Execution timing differs and it matters.
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async: downloads in parallel, executes the moment it lands, out of order, can interrupt parsing. Good for independent tags (analytics).
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defer: downloads in parallel, executes after HTML parse, in order. Good for app code with dependencies.
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Mistake: async on your main bundle. It fires mid-parse, blocks the main thread, INP spikes ~+120ms during load.
Use defer for ordered app JS, async only for fire-and-forget.
Takeaway: defer protects parse + order; async risks +100ms INP on app code.