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Upcoming: prioritize critical css & inserting dns prefetch

Monday, May 27, 2013

Our upcoming IISpeed release will have two brand new rewrite filters to speed up you website. Our next release is build on top of the latest version of Google's PageSpeed Optimization Libraries, which amongst a lot of other fixes and improvements brings these two awesome new filters:

Prioritize Critical CSS

This rewriter will determine which css rules are actually needed to display the page, and will inline just that. Once it has done that, it will inline just that part of the css, and defer loading of the rest until after the page has been loaded.

Doing so greatly helps with speeding up page rendering. Normally, the browser will have to wait until all css is loaded before it can start rendering the page. With this filter however, the necessary css will be available almost immediately. This one can shave seconds on mobile browsers!

To determine which css rules are needed for the initial view of a page, IISpeed injects javascript in the page, which will use beaconing to report statistics back to IISpeed. Because of this, this rewriter will only become fully effective after a page has been requested a few times.

Insert DNS Prefetch

This rewriter will scan pages for domains that browsers will need to resolve to be able to download resources from them. Once the rewriter has collected these domains, it uses that knowledge to insert prefetch entries into the html head. Doing so enables browsers to start resolving these dns entries very early, which has a potential to decrease the total time to rendering the page considerably. If you use a lot of external content, this rewriter is a must have!

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