Render-blocking resources
Style and script files the browser must download and process before it can show the page on screen.
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Why it matters
Every style file loaded at the top of the page and every script it waits for delays the first content. Inlining critical styles, loading the rest later and deferring scripts shorten this wait. The effect is especially visible on mobile devices and slow connections.
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Example
A chat widget script loaded at the top of a site delays the page appearing by a second. Once the script is deferred, the page appears immediately and the chat button arrives a few seconds later.
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Common mistake
Deferring all styles. Without critical styles the page first appears unstyled and then changes suddenly; styles needed for the first screen must load immediately.
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Related terms
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