Which CSS is faster?

Inline CSS is generally faster due to the elimination of extra HTTP requests. This is because the browser doesn't need to download an external stylesheet.

Let's break down why:

The Rendering Process and CSS

Browsers parse HTML and construct the DOM (Document Object Model). They also encounter CSS, which is crucial for styling the page. Critically, CSS blocks rendering. This means the browser won't paint anything to the screen until the CSSOM (CSS Object Model) is constructed.

Inline CSS vs. External CSS

Inline CSS: This CSS is written directly within HTML elements using the style attribute. Since the CSS is embedded within the HTML, the browser doesn't need to make an additional HTTP request to fetch a separate CSS file.
```
<p style="color: blue; font-size: 16px;">This is a blue paragraph.</p>
```
External CSS: This CSS is stored in separate .css files, which are then linked to the HTML document. The browser must download these files to apply the styles.
```
<head>
  <link rel="stylesheet" href="styles.css">
</head>
```

Why Inline CSS Can Be Faster: The Network Request

The key difference lies in the network request. Each external CSS file requires the browser to:

Make a Request: The browser sends a request to the server for the CSS file.
Receive a Response: The server responds with the CSS file content.
Parse the CSS: The browser parses the CSS to construct the CSSOM.

Inline CSS eliminates steps 1 and 2. The CSS is already present within the HTML, ready to be parsed. This reduces latency and can improve the perceived page load time, especially for small amounts of styling.

Trade-offs: Why External CSS Is Usually Preferred

While inline CSS can be faster in certain scenarios, it's generally not recommended for large projects for the following reasons:

Maintainability: Inline CSS makes it difficult to maintain a consistent style across your website. You'd need to update the style in every individual element, which is prone to errors.
Code Duplication: Inline CSS leads to code duplication, increasing the overall size of the HTML file and making it harder to read.
Caching: External CSS files can be cached by the browser, meaning subsequent page loads won't require downloading the CSS again. Inline styles cannot be cached in the same way.
Content Security Policy (CSP): Inline styles can be restricted by CSP for security reasons, adding complexity.

When Inline CSS Makes Sense

Critical CSS: Inlining critical CSS (the CSS needed for the above-the-fold content) can significantly improve perceived performance. This allows the user to see content faster while the rest of the CSS loads in the background. Tools and techniques exist to automate the extraction and inlining of critical CSS.
Very Small Projects: For very simple websites with minimal styling, inline CSS might be acceptable.
Dynamically Generated Styles: In some cases, styles might be generated dynamically on the server and injected directly into the HTML.

The Hybrid Approach: Critical CSS and External Stylesheets

The most common and effective approach is to combine inline critical CSS with external stylesheets. This provides the benefits of both: fast initial rendering and maintainable, cacheable stylesheets for the rest of the page.

Conclusion

Inline CSS can be faster because it avoids an HTTP request, but external CSS is generally preferred for maintainability, caching, and scalability. The optimal approach often involves inlining critical CSS and using external stylesheets for the rest of the site's styling.

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