CSS play button

play.css

.play {
    position: relative;
    z-index: 1;
    width: 100%;
    height: 100%;
    }
    
.play:before { /* triangle */
    content: "";
    display: block;
    position: absolute;
    z-index: 3;
    top:  50%;
    left: 50%;
    margin-top:  -10px;
    margin-left: -7px;
    border-top:    10px solid transparent;
    border-bottom: 10px solid transparent;
    border-left:   20px solid white;
    }
    
.play:after { /* circle */
    content: "";
    display: block;
    position: absolute;
    z-index: 2;
    width:  40px;
    height: 40px;
    top:  50%;
    left: 50%;
    margin-top:  -24px;
    margin-left: -24px;
    background: rgba(0,0,0,0.5);
    border: 4px solid white;
    -webkit-border-radius: 50%;
    -moz-border-radius: 50%;
    border-radius: 50%;
    -webkit-box-shadow: 0px 0px 10px black;
    -moz-box-shadow: 0px 0px 10px black;
    box-shadow: 0px 0px 10px black;
    }
    

play.html

<div class="play">
  <img src="video.jpg" />
</div>