interfering with tag

#Syllabus a is a descendant selector. That means the rule is applied to all anchors that are descendant of #Syllabus.

Using a child selector would select only anchors that are direct children of #Syllabus: #Syllabus > a.

#Syllabus > a {
    position:          absolute;
    top:               0px;
    left:              130px;
    width:             112px;
    height:            40px;
    background-image:  url(/Assets/Syb_but.png);
    background-repeat: no-repeat;
}

#Syllabus > a:hover {
    background-image: url(/Assets/QuesPap_but_ul.png);
}

.ul_menu {
    position: relative;
    top:      44px;
    left:     43px;
    display:  none;
}

ul, li {
    margin:  auto;
    padding: 0;
}