Even ignoring the Player / AIR runtime, Flash is a fantastic tool for animation authoring. For example, our game artists use Flash to create SWF clips which we then export and playback with an internal format and C++ engine, supporting a fairly limited subset of all SWF features. This content ranges from UI layouts and transition animations to particle special effects and storytelling cutscenes.
It wasn't hard to write it myself (I'd guess about 40 hours for everything), but if we were going to make HTML5 games, I'd definitely consider this library.
It wasn't hard to write it myself (I'd guess about 40 hours for everything), but if we were going to make HTML5 games, I'd definitely consider this library.