.yblist {
position: absolute;
width: 100%;
height: 100%;
background: #000;
opacity: 0.9;
z-index: 999;
}
.flex {
display:-webkit-box;
display:-ms-flex;
display:-webkit-flex;
display:flex;
}
.flex-hc {
-webkit-box-pack:center;
-ms-justify-content:center;
-webkit-justify-content:center;
justify-content:center;
}
.flex-vc {
-webkit-box-align:center;
-ms-align-items:center;
-webkit-align-items:center;
align-items:center;
}
.transCenter {
position: absolute;
left:50%;
top:50%;
-webkit-transform:translate(-50%,-50%);
transform:translate(-50%,-50%);
}
<div class="yblist flex flex-hc flex-vc">
<span style='color:#FCF'>21</span>
</div>
<div class="yblist">
<span style='color:#FCF' class='transCenter'>21</span>
</div>