I forgot to mention the myriad ways that flash would interact with the browser and the operating system, with swfs often designed to interact with browser javascript in particular, in platform and browser specific ways. the various network protocols, RTMP streaming, prioprietary video codecs, ALPHA MASKED video, the CSS parser and styling engine! the API has everything but the kitchen sink. You can have a look at the sorts of challenges you're in for by looking at Shumway's issues register here:
https://github.com/mozilla/shumway/issues