<main>
<nav></nav>
<section></section>
</main>
nav
width: 50px
position: fixed
top: 0
bottom: 0
left: 0
background: #151515
nav:hover
width: 150px
section
position: fixed
top: 0
right: 0
bottom: 0
left: 70px
background: #151515
nav:hover section
left: 170px