Load scripts with defer, async, or type=module
A plain <script> tag in the document head blocks all HTML parsing until the script downloads, parses, and executes. On a slow network this can add seconds of white-screen time before any content renders. defer and async allow the browser to continue parsing HTML while the script downloads, reducing Time to First Contentful Paint dramatically.
Quick Reference
- defer downloads in parallel and executes after HTML parsing, in order — use for most scripts
- async downloads in parallel and executes immediately when ready — use for independent scripts
- type=module always defers and enables ES module syntax
- Never place scripts in <head> without defer or async
Check
Find all script tags in this HTML file. Flag any in the <head> without defer or async, and any at the bottom of <body> that could be in <head> with defer.
Fix
Add defer or async to script tags in the document head, or convert to type=module where ES modules are used.
Explain
Explain the difference between defer, async, and type=module script loading, and when to use each.
Code Review
Review templates, server-rendered HTML, and shared components …
