问一个非常非常奇怪的CSS/ASP.NET错误

EN

/*Basic CSS menu Layout*/
#Menu
{
    font-size: 24pt;
    height: 48px;
    width: 1000px;
    color: White;
    background-color: Black;
}

#Menu a
{
    color: White;
    text-decoration: underline;
    width: 224px;
    height: 48px;
}

#Menu a:visited
{
    color: White;
    text-decoration: none;
}

#Menu a:hover
{
    color: White;
    font-style: italic;
}
/*StyleSheetTheme menu layout*/
#Menu a
{
    background-image: url('Menu_Default.png');
    background-repeat: no-repeat;
}

#Menu a:hover
{
    background-image: url('Menu_Default.png');
    background-repeat: no-repeat;
}

#Menu a:visited
{
    background-image: url('Menu_Default.png');
    background-repeat: no-repeat;
}

#Menu a:active
{
    background-image: url('Menu_Default.png');
    background-repeat: no-repeat;
}