Flash is supported in 93% to 99.9% of the browsers out there (all 4+ browsers and several older ones) as long as you avoid action scripting beyond those parts that were supported in Flash 3. You should still save to Flash 4 however as it compresses better. And split your movie up into several smaller files using load movie. If you use complex action scripts the figure drops to somewhere between 60% to 80% of the web having Flash 4 plugin support.

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