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Product Manuals and Release notes
When a CSS link appears at the top of the page, preceding the </head> element, a browser can progressively render the page to quickly display the content, especially beneficial for users who access content-rich pages by means of slower Internet connections.
When a JS link appears at the end of the page, preceding the </body> element, a browser can download multiple components in parallel for each hostname and accelerate the perceived page rendering. You can specify each JS link that you prefer to relocate to the end of the page, and, consequently, accelerate the perceived page rendering. Exceptions to reordering JS information include JS URLs and scripts that use document.write to insert content for the page.