<style>
.box {
width: 200px;
height: 300px;
background: red;
position: relative;
}
.el1 {
height: 100px;
background: green;
}
.el2 {
background: blue;
width: 200px;
position: absolute;
top: 100px;
bottom: 0;
}
</style>
<div class="box">
<div class="el1"></div>
<div class="el2"></div>
</div>
<style>
.box {
width: 200px;
height: 300px;
background: red;
}
.el1 {
height: 100px;
background: green;
}
.el2 {
height: calc(100% - 100px);
background: blue;
}
</style>
<div class="box">
<div class="el1"></div>
<div class="el2"></div>
</div>