X = x0 + (x2-x1)
 Y = y0 + (y2-y1)

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;"></div>
<script>
test.onmousedown = function(e){
 e = e || event;
 //获取元素距离定位父级的x轴及y轴距离
 var x0 = this.offsetLeft;
 var y0 = this.offsetTop;
 //获取此时鼠标距离视口左上角的x轴及y轴距离
 var x1 = e.clientX;
 var y1 = e.clientY;

 test.onmousemove = function(e){
  e = e || event;
  //获取此时鼠标距离视口左上角的x轴及y轴距离
  x2 = e.clientX;
  y2 = e.clientY; 
  //计算此时元素应该距离视口左上角的x轴及y轴距离
  var X = x0 + (x2 - x1);
  var Y = y0 + (y2 - y1);
  //将X和Y的值赋给left和top，使元素移动到相应位置
  test.style.left = X + 'px';
  test.style.top = Y + 'px';
 }

 test.onmouseup = function(e){
  //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
  test.onmousemove = null;
 }
}
</script>

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;"></div>
<script>
test.onmousedown = function(e){
 e = e || event;
 //获取元素距离定位父级的x轴及y轴距离
 var x0 = this.offsetLeft;
 var y0 = this.offsetTop;
 //获取此时鼠标距离视口左上角的x轴及y轴距离
 var x1 = e.clientX;
 var y1 = e.clientY;

 document.onmousemove = function(e){
  e = e || event;
  //获取此时鼠标距离视口左上角的x轴及y轴距离
  x2 = e.clientX;
  y2 = e.clientY; 
  //计算此时元素应该距离视口左上角的x轴及y轴距离
  var X = x0 + (x2 - x1);
  var Y = y0 + (y2 - y1);
  //将X和Y的值赋给left和top，使元素移动到相应位置
  test.style.left = X + 'px';
  test.style.top = Y + 'px';
 }

 document.onmouseup = function(e){
  //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
  document.onmousemove = null;
 }
}
</script>

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">测试文字</div>
<script>
test.onmousedown = function(e){
 e = e || event;
 //获取元素距离定位父级的x轴及y轴距离
 var x0 = this.offsetLeft;
 var y0 = this.offsetTop;
 //获取此时鼠标距离视口左上角的x轴及y轴距离
 var x1 = e.clientX;
 var y1 = e.clientY;

 document.onmousemove = function(e){
  e = e || event;
  //获取此时鼠标距离视口左上角的x轴及y轴距离
  x2 = e.clientX;
  y2 = e.clientY; 
  //计算此时元素应该距离视口左上角的x轴及y轴距离
  var X = x0 + (x2 - x1);
  var Y = y0 + (y2 - y1);
  //将X和Y的值赋给left和top，使元素移动到相应位置
  test.style.left = X + 'px';
  test.style.top = Y + 'px';
 }

 document.onmouseup = function(e){
  //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
  document.onmousemove = null;
 }
 //阻止默认行为
 return false;
}
</script>

<button id="btn1">按钮一</button>
<button id="btn2">开启按钮一的全局捕获</button>
<script>
btn1.onclick = function(){
 alert(1);
}
btn2.onclick = function(){
 if(btn1.setCapture){
  if(btn2.innerHTML.charAt(0) == '开'){
   btn1.setCapture();
   btn2.innerHTML = '关闭按钮一的全局捕获';
  }else{
   btn1.releaseCapture();
   btn2.innerHTML = '开启按钮一的全局捕获'; 
  }
 }
}
</script>

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">测试文字</div>
<script>
test.onmousedown = function(e){
 e = e || event;
 //获取元素距离定位父级的x轴及y轴距离
 var x0 = this.offsetLeft;
 var y0 = this.offsetTop;
 //获取此时鼠标距离视口左上角的x轴及y轴距离
 var x1 = e.clientX;
 var y1 = e.clientY;

 document.onmousemove = function(e){
  e = e || event;
  //获取此时鼠标距离视口左上角的x轴及y轴距离
  x2 = e.clientX;
  y2 = e.clientY; 
  //计算此时元素应该距离视口左上角的x轴及y轴距离
  var X = x0 + (x2 - x1);
  var Y = y0 + (y2 - y1);
  //将X和Y的值赋给left和top，使元素移动到相应位置
  test.style.left = X + 'px';
  test.style.top = Y + 'px';
 }

 document.onmouseup = function(e){
  //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
  document.onmousemove = null;
  //释放全局捕获
  if(test.releaseCapture){
   test.releaseCapture();
  }
 }
 //阻止默认行为
 return false;
 //IE8-浏览器阻止默认行为
 if(test.setCapture){
  test.setCapture();
 }
}
</script>