Until each of their 4.0 Beta 2 versions, both Netscape and Internet Explorer have not allowed an animated Gif to be used as a background image. Sometimes only the first frame of the animation would be shown - otherwise no image at all. Warning: even though background animations are now possible, PLEASE exercise caution in their use! This behavior could be many more times as annoying (and headache causing =)) to readers than special effects such as BLINK or MARQUEE ever have been.
This document contains an animated GIF background image reference
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