What if I want to use html+css+js to
    dynamically draw a 1024x1024 picture of a cat

Here is your cat:

http://upload.wikimedia.org/wikipedia/commons/f/fd/Ghostscri...

    a renderer that has bezier curves and lines

SVG is part of the HTML standard and has those. And much more:

http://www.google.com/search?q=svg+examples