YSlow: Avoid AlphaImageLoader filter
AlphaImageLoader blocks rendering and increases page load times.
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Details from Yahoo!
AlphaImageLoader filter attempts to fix a problem with semi-transparent true color PNG files in IE versions less than Version 7. However, this filter blocks rendering and freezes the browser while the image is being downloaded.
Additionally, it increases memory consumption. The problem is further multiplied because it is applied per element, not per image.