\"onMouseUp\" vent virker ikke

I min <td> har jeg en række eventhandlers , som egentligt virker ok, bortset fra \"onMouseUp\". Hvorfor det? (\"onMouseUp\" er når man slipper den venstre musseknap efter at have trykket den ned, ik\'?)

<td id=\"oBold\" class=\"btn\" align=\"center\" onClick=\"changeBtnClass_Down(this); document.execCommand(\'Bold\');\" onMouseUp=\"changeBtnClass_Up(this)\"; onMouseOver=\"changeBtnClass_Over(this);\" onMouseOut=\"changeBtnClass_Out(this);\">
  <b>B</b>
</td>

Functions:

function changeBtnClass_Over(element) {
  element.className = \"btnOver\";
}

function changeBtnClass_Up(element) {
  element.className = \"btnOver\";
}

function changeBtnClass_Out(element) {
  element.className = \"btn\";
}

function changeBtnClass_Down(element) {
  element.className = \"btnDown\";
}


Styles:

.btn {
  background-color: buttonface;
  border-bottom: buttonface 1px solid;
  border-left: buttonface 1px solid;
  border-right: buttonface 1px solid;
  border-top: buttonface 1px solid;
  height: 23px;
  top: 1px;
  width: 23px;   
}

.btnOver {
  background-color: buttonface;
  border-bottom: buttonshadow 1px solid;
  border-left: buttonhighlight 1px solid;
  border-right: buttonshadow 1px solid;
  border-top: buttonhighlight 1px solid;
  height: 23px;
  top: 1px;
  width: 23px;
}

.btnDown {
  background-color: buttonface;
  border-bottom: buttonhighlight 1px solid;
  border-left: buttonshadow 1px solid;
  border-right: buttonhighlight 1px solid;
  border-top: buttonshadow 1px solid;
  height: 23px;
  top: 1px;
  width: 23px;
}