Shumway loads, parses and executes SWF with JS, and uses HTML5 <canvas>
to render it.
You can see a bit better what's going on behind the scenes when you open the SWF in the Shumway Inspector. Direct link for the racing game: http://www.areweflashyet.com/shumway/examples/inspector/inspector.html?rfile=../racing/race.swf