Wired margin-top disappear

by Lien Z

HTML

<div class='head-container' id="headerCom">
    <header id="a"></header>
</div>
<div id="content" role="main"></div>

CSS

* {
    margin: 0;
    padding: 0
}
body {
    width: 100%;
    height: 100%;
}
.head-container {
    position: fixed;
    top:0;
    left:0;
    width: 100%;
    height: 100px;
    background: red;
    _position:absolute;
    _top: expression(eval(document.documentElement.scrollTop));
}
header {
    display: block;
    width: 960px;
    height: 100px;
    margin: 0 auto;
    position: relative;
    zoom: 1;
    background: blue;
}
#content {
    border: 1px solid black;
    margin: 130px auto 0 auto;
    width: 960px;
    height: 1000px;
    background: #999;
    margin-top: 150px;
}