Processing, especially the javascript library is ready to go on HTML canvas.
This is as opposed to what you do nowadays; use one piece of software for drawing, possibly another for animating, then export those images and/or animations to some format, linkit together with a separate language and/or framework, and maintain assets separately of code. Flash pulled all of these things into one, but in a way that allowed for really complex projects to be made.
A litany of tools for manipulating images and building animations right in the program, without needing to write a single line of code.
And then, on top of that, you can actually.... write code! And it works quite well!
Flash is one of the funnest programming environment to use for "interactive art". Straightforward things are almost always straightforward, and it's much easier to WYSIWYG your way through a lot of tedium.