Screen centering block

Based on vertical-allign

HTML

<div class="full container fixed">
    <div class="modal">
      <h1>
       Here!
      </h1>
      <div>
        Is contenent here!
      </div>
    </div>
</div>

CSS

.full { width:100%; height:100%;}
    .fixed { position: absolute; } /* здесь может быть как fixed */
.container { display: block;  text-align: center; vertical-align:center; line-height:100vh; }
    .modal { display: inline-block; line-height:1em; outline:1px solid green; }