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 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.