真是奇怪, 究竟是为什么?

<?xml version="1.0" standalone="no"?>

<svg onload="init()">
  <!-- script type="text/javascript" xlink:href="train.js"/ -->
  <script type="text/javascript"><![CDATA[
    var svgdoc;
    var newCircle;
    var newMotion;
    var newPath;
    var test;
    var svgns="http://www.w3.org/2000/svg";

    function init()
    {
      svgdoc = document;
      test = svgdoc.getElementById("test");

      go();
    }

    function go()
    {
      addMotion();
    }

    //增加一个运动的圆
    function addMotion()
    {
      newCircle=svgdoc.createElementNS(svgns,"circle");
      newCircle.setAttribute("cx","0");
      newCircle.setAttribute("cy","0");
      newCircle.setAttribute("r","5");
      test.appendChild(newCircle);

      newMotion = svgdoc.createElementNS(svgns,"animateMotion");
      newMotion.setAttribute("begin","1s");
      newMotion.setAttribute("dur","5s");
      //如果使用下面的属性，增加的圆可以运动
      //newMotion.setAttribute("path","M100,100 T200,200");
      newCircle.appendChild(newMotion);

      //使用mpath元素就不能运动
      newPath = svgdoc.createElementNS(svgns,"mpath");
      newPath.setAttribute("xlink:href","#path1");
      newMotion.appendChild(newPath);
    }

    function Copy2Clipboard()
    {
      var shapes = test.childNodes;
      var s = "";

      if(top.clipboardData && shapes.length>0)
      {
        for(i=0;i<shapes.length;i++)
        {
          s+="\r\n  "+printNode(shapes.item(i));
        }
        top.clipboardData.setData("Text",s);
      }
    }

  ]]></script>

  <defs>
    <circle id="111" cx="100" cy="100" r="5" style="fill:white;stroke:black"/>
    <circle id="222" cx="200" cy="200" r="5" style="fill:white;stroke:black"/>
    <path id="path1" d="M100,100 T200,200" style="fill:none;stroke:black;stroke-width;3"/>
  </defs>

  <use xlink:href="#path1"/>
  <use xlink:href="#111"/>
  <use xlink:href="#222"/>

  <!--  在此增加一个运动的圆 -->
  <g id="test">
  <!-- circle cx="0" cy="0" r="5"><animateMotion begin="1s" dur="5s" accumulate="none" additive="replace" calcMode="linear" fill="remove" restart="always"><mpath xlink:href="#path1"/></animateMotion></circle> -->
  </g>

  <circle cx="0" cy="0" r="5" fill="#F00">
    <animateMotion begin="0s" dur="5s" accumulate="none" additive="replace" calcMode="linear" restart="always">
      <mpath xlink:href="#path1"/>
    </animateMotion>
  </circle>

  <a xlink:href="">
    <text x="5" y="390" onclick="Copy2Clipboard()">&gt; Copy
      <set attributeName="fill" attributeType="CSS" to="#F00" begin="mouseover"/>
      <set attributeName="fill" attributeType="CSS" to="#000" begin="mouseout"/>
    </text>
  </a>
</svg>