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What is “Render-blocking JavaScript & CSS”

Before browsers render any content from your websites, they need to parse the HTML markup into a DOM tree. The HTML parser is paused by render-blocking resources — any external stylesheets or synchronous JavaScript tags that it encounters.

Scripts ( <script src="main.js"> ) and stylesheets ( <link rel="stylesheet"> ) are both render blocking resources which delay your page from showing your content to visitors (See: FCP and LCP). It is recommended that any non-critical JavaScript or CSS is deferred to speed up the loading of your website’s main content.

Pagespeed Insights suggestions to eliminate render-blocking resources on CVS.com
Pagespeed Insights suggestions to eliminate render-blocking resources on CVS.com

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