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Stylesheets Block Downloads in Firefox and JavaScript Execution in IE
posted by Steve Souders, March 17, 2008 2:10 PM
In Firefox, stylesheets will block all other downloads. This is especially bad for external scripts, because stylesheets and external scripts are frequently placed near each other in the page. This page contains a stylesheet (2 second download), an external script (1 second download, 1 second execution), and an image (2 second download). As shown in the Firefox HTTP profile below, all of these items load sequentially making the total page load in a little over 6 seconds. Notice the white gap after the script and before the image; this is the 1 second script execution time.

In IE stylesheets do not block other downloads. But they do block JavaScript execution. In the IE HTTP profile below we see that the stylesheet and script start downloading in parallel. The script finishes downloading first (it's only a 1 second download while the stylesheet is a 2 second download), but then there is a 1 second white gap while the script executes, and finally the image is downloaded. Because the stylesheet and script load in parallel at the beginning, the page loads in 5 seconds (1 second faster than Firefox).

    </HEAD> <BODY>
      </BODY> </HTML>