Fixed at side

HTML

<!-- Fixed at the middle of left side -->
<div class="fixed-at-side fixed-at-side--l">...</div>

<!-- Fixed at the middle of right side -->
<div class="fixed-at-side fixed-at-side--r">...</div>

CSS

.fixed-at-side {
    position: fixed;
    top: 50%;
    transform: translate(0px, -50%);
}
.fixed-at-side--l {
    left: 0;
}
.fixed-at-side--r {
    right: 0;
}