center

.mask{
position:absolute;left:0;top:0;height:100vh;100vw;filter:Alpha(opacity=60);background-color: rgba(0,0,0,0.6);font-size:3vh;

display: -ms-flexbox;
-ms-flex-align:center;

display: -webkit-box;
-webkit-box-orient: horizontal;
-webkit-box-pack: center;
-webkit-box-align: center;
display: box;
box-orient: horizontal;
box-pack: center;
box-align: center;

display: -webkit-flex;
display: flex;
-webkit-align-items: center;
align-items: center;
-webkit-justify-content: center;
justify-content: center;
100%;
height: 100%;
}