.a {
background: #bbb;
height: 4rem;
line-height: 4rem;
font-size:0;
display:inline-block;
}
.b {
background: #888;
display: inline-block;
height: 1.5rem;
line-height: 1.5rem;
font-size: 0.5rem;
vertical-align: middle;
}
position: absolute;
top: 50%;
transform: translateY(-50%);