Measure CSS rendering time

The Net in Firebug can do this.

Enable it, reload the page, and click on the CSS subtab.

There are two values you can look as the render completion times, the thin red and thin blue lines, compared to the end of the loading of actual files. DOMContentLoaded is the vertical blue line and load is represented by the vertical red line.

You can see the exact values by hovering over any request. (See screenshot.)

See http://getfirebug.com/wiki/index.php/Net_Panel

Also: http://www.webpagetest.org/