🧠 ct.css – Let’s take a look inside your <head>

Computed tomography of the head uses a series of X-rays in a CT scan of the head…
— wikipedia.org/Computed_tomography_of_the_head

Your <head> is the single biggest render-blocking part of your page—ensuring it is well-formed is critical. ct.css is a diagnostic CSS snippet that exposes potential performance issues in your page’s <head> tags.

Link to presentation deck

Your optimal goal:

Example Output

• Red: This is an error and should be addressed.
• Orange: This could be problematic in certain scenarios.
• Green: This is fine and is purely informational.
• Solid: This file is the problem.
• Dashed: Another file(s) are causing problems with this file.

Simple Usage

Paste this anywhere in your HTML:

<link rel="stylesheet" href="https://csswizardry.com/ct/ct.css" class="ct" />

Chrome Snippet

Run Snippets Of JavaScript On Any Page With Chrome DevTools

(function(){
  var ct = document.createElement('link');
  ct.rel = 'stylesheet';
  ct.href = 'https://csswizardry.com/ct/ct.css';
  ct.classList.add('ct');
  document.head.appendChild(ct);
}());

Limitations

• Non-matching media types: Currently, ct.css only takes print stylesheets into account. This means that you may experience false-positives for other non-blocking stylesheets.
• Injected script elements: ct.css, as with all CSS, acts against the DOM and not the HTML. As such, injected script elements will be falsely flagged as blocking.
  • Similarly, any script elements that are removed from the DOM will not be flagged.
• Third-party blocking resources: The check for whether or not something is a third party is naive at best.
• CSP Issues: If your app is blocking CSS from the csswizardry.com origin, either self-host the ct.css file, or paste its contents into <style class="ct">…</style> tags anywhere in your page.