<!DOCTYPE html>
<style>
#container div {
height: 50px;
border: 1px solid black;
margin-top: 2px;
}
#container {
border: 1px solid green;
height: 500px;
}
#container #first {
margin-top: 100px;
background: yellow;
}
</style>
<div id="container">
<div id="first"></div>
<div></div>
<div style="height: 51px"></div>
<div></div>
<div></div>
<div></div>
<div></div>
</div>