On the SWF features that Swiffy would support, this is what the FAQ says:
Swiffy supports many common SWF features such as vector graphics, embedded fonts, images and timeline animation. Basic ActionScript 2.0 code is also supported, but don't expect to convert your favorite Flash game yet. In general, Swiffy supports most of the features in Flash 5, so exporting your file as a Flash 5 will give the best results. The Swiffy compiler will warn you when unsupported SWF features are encountered.
Adobe also has a tool called Wallaby that converts Flash FLA to HTML. The main difference between Wallaby and Swiffy is that the former is an installable tool and the latter a web-based tool. Also, Swiffy's generated format isn't as easily editable.