SVG, 177 bytes
<svg><g transform=translate(75,75)><circle id=x r=0.5 cx=56 fill=#800080><animateTransform xlink:href=#x attributeName=transform type=rotate to=360 dur=6s repeatCount=indefiniteInvalid markup from hell to breakfast, but it runs (in Chrome at least). Like a HTML5 canvas, the default size for an SVG appears to be 300x150, so that's what this is assuming.