CSS Performance Optimization Best Practices
CSS affects page rendering speed more than developers realize. Learn how to reduce render-blocking, optimize selectors, and minimize layout thrashing.
Key Takeaways
- Browsers won't render content until they've downloaded and parsed all CSS.
- CSS selectors are evaluated right-to-left.
- Remove unused CSS with tree-shaking tools.
- Reading layout properties (offsetHeight, getBoundingClientRect) forces the browser to recalculate layout.
- Custom fonts can cause FOIT (Flash of Invisible Text) or FOUT (Flash of Unstyled Text).
CSS Minifier
CSS Is Render-Blocking
Browsers won't render content until they've downloaded and parsed all CSS. A large stylesheet delays the first paint. Critical CSS (styles for above-the-fold content) should be inlined in for fastest first paint.
Selector Performance
CSS selectors are evaluated right-to-left. A selector like .sidebar .nav li a first finds all elements, then filters. Flat selectors (.nav-link) are faster than deeply nested ones.
Reducing CSS Size
- Remove unused CSS with tree-shaking tools.
- Use shorthand properties (
margin: 0 autoinstead of four margin declarations). - Combine duplicate rules.
- Use modern features like
gapinstead of margin hacks.
Layout Thrashing
Reading layout properties (offsetHeight, getBoundingClientRect) forces the browser to recalculate layout. Batch reads and writes separately to avoid forced synchronous layout.
Font Loading
Custom fonts can cause FOIT (Flash of Invisible Text) or FOUT (Flash of Unstyled Text). Use font-display: swap to show fallback fonts immediately while custom fonts load.
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