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Get Started Free →Use when reviewing templates, rendered HTML, or shared components related to Load scripts with defer, async, or type=module. Validate the final browser-facing markup, not just the source framework abstraction.
| Test case | Without → With | Effect | Δ tokens | Δ turns |
|---|---|---|---|---|
| case-01 | ✗→✓ | ▲ Improved | 82% | 0% |
| case-02 | ✓→✓ | ＝ Same ✓ | -1% | 0% |
| case-03 | ✓→✓ | ＝ Same ✓ | 9% | 0% |
| case-04 | ✓→✓ | ＝ Same ✓ | -9% | 0% |
| case-05 | ✓→✓ | ＝ Same ✓ | -24% | 0% |
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.
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.
Add defer or async to script tags in the document head, or convert to type=module where ES modules are used.
Explain the difference between defer, async, and type=module script loading, and when to use each.
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
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