middle virker ikke

hvorfor virker vertical-align: middle; ikke i mine tabeller med hover effekt?

<style type="text/css">
<!--
a.menu1 {
    text-decoration: none;
    background : #FFFFFF center center;
    display:block;
    width:100%;
    border-right: 1px #cccccc solid;
    border-left: 0px #cccccc solid;
    border-top: 0px;
    border-bottom: 0px;
    width: 90px;
    height: 18px;
    text-align: center;
    font-family: verdana;
    font-size: 9px;
    color: #2C4356;
    vertical-align: middle;
 
}

a.menu1:hover {
text-decoration: none;
background : #EFEFEF
display:block;
width:100%;
border-right: 1px #cccccc solid;
  border-left: 0px #cccccc solid;
  border-top: 0px;
  border-bottom: 0px;
  width: 90px;
  height: 18px;
  text-align: center;
  vertical-align: middle;
}
-->