title: Canvas offsetTop and offsetLeft
body: <p>In HTML5 Canvas what is offsetTop and offsetLeft ?</p>
  
  <p>I'm trying to get the X and Y of a a mouse click event. I know I can get that through:</p>
  
  <pre><code>mycanvas.onclick = function (evt) {
      var offX = evt.layerX - mycanvas.offsetLeft;
      var offY = evt.layerY - mycanvas.offsetTop;
  }
  </code></pre>
  
  <p>but what is offsetLeft and offsetTop? and what is LayerX and LayerY ?</p>