Click the green flag to play the animation (or to replay it.)
You may also have to click anywhere on the animation
before it will respond to spacebar presses.
To stop the animation click the red "stop sign".
The source code of this page attempts to demonstrate how to host a scratch.sb file on an HTML5 page. I've heard there are problems when hosting an .sb animation in HTML5 pages. I've found that they don't run online, though they do run locally.
My experiments work fine when hosting .sb Scratch files via <applet> tags on HTML, XHTML or HTML5 pages, locally. The only other requirement is to load ScratchApplet.jar and soundbank.gm in the same webhost folder.