I would start by looking at the Raphael library .
This allows you to write fairly simple Javascript code to draw graphic elements in the browser, including fairly simple animations and drag / drop interfaces.
Honestly, itโs not easy and simple to write something complicated, like the site you linked to, but Raphael is the fastest way to get started with the basics.
And best of all, Raphael is even compatible with older versions of IE, so you donโt only have to worry about supporting the latest browsers (although, of course, you may want to do other things that need other new features).
Hope this helps.
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