Defer Async
Testing & Qualitydefer-asyncDefer Async is an Agent Skill for testing & quality published in the thedaviddias/Front-End-Checklist repository on GitHub: a SKILL.md instruction file an AI agent can load to do this task.
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.
How to use this skill
Bring this guide into your coding agent with a prompt tailored to the tool you use.
- Open your project in Codex.
- Copy and paste the prompt below.
- Review files and risks before approving any action.
Inspect the Agent Skill at https://github.com/thedaviddias/Front-End-Checklist/blob/HEAD/skills/defer-async/SKILL.md and all supporting files. Report provenance, permissions, scripts, dependencies, and security risks. Propose an exact installation into .agents/skills/defer-async, but wait for my approval before writing files or running scripts.
Copying this prompt does not install or run the skill. Review third-party files before use.
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.
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