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 without defer or async
Check
Find all script tags in this HTML file. Flag any in the without defer or async, and any at the bottom of that could be in 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 that output markup related to Load scripts with defer, async, or type=module. Flag exact elements, attributes, and routes where the rendered HTML violates the rule.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
Rule page: https://frontendchecklist.io/en/rules/html/defer-async