<div id="div1">
<span id="sp1">aaa</span>
</div>
#div1{
min-height:100px;
background-color: yellow;
position: relative;
}
#sp1{
width: 20%;
height: 100%;
background-color: blue;
position: absolute;
}
<div id="div1">
<span id="sp1">aaa</span>
</div>
#div1{
min-height:100px;
background-color: yellow;
position: relative;
}
#sp1{
width: 20%;
height: 100%;
background-color: blue;
position: absolute;
}