What are the key takeaways from this guide?

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)..

Who is this guide for?

This guide is designed for beginner-level users and takes about 1 minutes to read.

Best Practice Beginner 1 min read 159 words

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).

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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 auto instead of four margin declarations).
Combine duplicate rules.
Use modern features like gap instead 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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