Render-blocking JavaScript and CSS are common culprits behind slow page loads. Here's how to optimize your code for better performance.
Understanding Render-Blocking Resources
By default, browsers must download and parse CSS and JavaScript before rendering the page. This delays when users see content.
CSS Optimization
1. Minify CSS
Remove whitespace, comments, and unnecessary characters. Tools: cssnano, clean-css, or your build tool.
2. Inline Critical CSS
Include above-the-fold styles directly in the HTML head. Load remaining CSS asynchronously.
3. Remove Unused CSS
Use tools like PurgeCSS to identify and remove unused styles. Many sites have 90%+ unused CSS.
4. Combine CSS Files
Reduce HTTP requests by combining multiple stylesheets (though HTTP/2 makes this less critical).
JavaScript Optimization
1. Defer Non-Critical JavaScript
<script src="script.js" defer></script>Defer downloads the file during HTML parsing but executes after parsing completes.
2. Async for Independent Scripts
<script src="analytics.js" async></script>Async downloads and executes as soon as possible, regardless of HTML parsing.
3. Minify JavaScript
Use Terser, UglifyJS, or your bundler's minification.
4. Code Splitting
Load only the JavaScript needed for the current page. Lazy-load components as needed.
5. Remove Unused JavaScript
Audit third-party scripts. Do you really need all of them?
Loading Strategies
| Attribute | Behavior | Use For |
|---|---|---|
| None | Blocks rendering | Critical scripts |
| defer | Executes after HTML parsing | Most scripts |
| async | Executes when ready | Analytics, ads |
Tools for Optimization
- Webpack/Rollup: Bundle and optimize
- Lighthouse: Identifies render-blocking resources
- Coverage tab: Shows unused CSS/JS in DevTools
Related: Core Web Vitals Explained
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