After many different versions of both Flash and ActionScript, Flash became more popular among web developers and viewers alike. Nearly everyone surfing the web had the Flash Player plugin installed, and sites using Flash began popping up left and right. Even after 2000, sites continued to use Flash; a famous example of this was YouTube, using Flash when it debuted in 2005.
Flash was later purchased by Adobe Systems, who contiued to develop it until somewhat recently. Flash changed web development just as past web technologies did. Flash made it easy for nearly anyone to make an animation and make it fit for viewing on the web. Sites were chock-full of big, flashy animations, links with insane hover and click effects, and vibrant colors.
It wasn't until HTML5's release in 2014 and CSS3's development in 2011 when Flash's popularity nearly dissapeared. With animations becoming possible using HTML5 and CSS3, Flash became less and less used, taking from its already its dwindling use. The new animations were faster, more semantic & straightforward, and generally the new, "right" way to do animations. Though Flash is essentially depreciated in most browsers now, its effects on web design were clear, and its use was a phase that web developers won't soon forget.