title: why CanvasRenderingContext2D is not defined in my IE9?
body: <p>I have a javascript HTML5 application based on prototype model, I extend CanvasRenderingContext2D of Canvas to support drawing ellipse and so on. </p>
  
  <p>Sudenly an error shows CanvasRenderingContext2D is undefined, and getContext is undefined either.</p>
  
  <p>What caused this problem?</p>