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highPerformance

Render-Blocking Scripts in Document Head

Scripts in <head> without defer/async block rendering, slowing page load and hurting Core Web Vitals.

By Seoxpert Editorial · Published · Updated

Why it matters

When scripts in the <head> lack defer or async, they pause HTML parsing until the script loads and executes. This delays the first contentful paint (FCP) and largest contentful paint (LCP), negatively impacting Core Web Vitals and user experience.

Impact

Leaving render-blocking scripts in the <head> slows down perceived page load and can lower SEO rankings.

How it's detected

Automated crawlers scan for <script> tags in <head> without defer or async attributes.

Common causes

  • Including third-party scripts in <head> without defer or async
  • Legacy code practices that place scripts in <head> by default
  • Lack of awareness about script loading attributes
  • Assuming all scripts must execute before page render

How to fix it

Add the defer attribute to non-critical scripts in the <head> to allow HTML parsing to continue. For scripts that must execute as soon as possible, use async. Alternatively, move scripts to just before the </body> tag. Example: <script src="app.js" defer></script>.

Code examples

Problem: Render-blocking script in <head>

<head>
  <script src="app.js"></script>
</head>

Fix: Add defer attribute to script

<head>
  <script src="app.js" defer></script>
</head>

Fix: Move script before </body>

<body>
  <!-- page content -->
  <script src="app.js"></script>
</body>

FAQ

Why do scripts in the <head> block rendering?

Scripts without defer or async pause HTML parsing until they load and execute, delaying page rendering.

Should all scripts use defer or async?

Most non-critical scripts should use defer. Use async only if script execution order doesn't matter and it's critical.

What if a script must run before the page renders?

Only truly critical scripts should block rendering; most can be deferred or loaded asynchronously.

Does moving scripts to the end of <body> solve the issue?

Yes, placing scripts before </body> prevents them from blocking initial rendering.

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