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https://github.com/csswizardry/ct

Let’s take a look inside your <head>…
https://github.com/csswizardry/ct

Last synced: 5 days ago
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Let’s take a look inside your <head>…

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# 🧠 `ct.css` – Let’s take a look inside your ``

> 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](https://en.wikipedia.org/wiki/Computed_tomography_of_the_head)

Your `` 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 `` tags.

## Example Output

![](./demo.png)

* **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:

```html

```

## Chrome Snippet

![](./chrome-snippet.png)

[_Run Snippets Of JavaScript On Any Page With Chrome DevTools_](https://developers.google.com/web/tools/chrome-devtools/javascript/snippets)

```
(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 `…` tags anywhere in your page.