demo.css

.vast-controls {
  width: 100%;
  display: flex;
  flex-direction: column;
}

.vast-test-btn {
  margin-top: 20px;
  border: none;
  box-shadow: 0 2px 5px 0 rgba(0,0,0,.26);
  padding: 8px 16px;
  border-radius: 5px;
  max-width: 100px;
  color: #fff;
  background: darkgray;
  font-family: Verdana, Geneva, Tahoma, sans-serif;
  font-size: 16px;
}

.vast-test-btn.active {
  background-color: #039be5;
  cursor: pointer;
}

.vast-media {
  position: relative;
  padding-top: 20px;
  display: flex;
  flex-flow: row wrap;
}

.vast-media video,
.video-ad-container {
  width:  100%;
  height: 100%;
}

.video-ad-container {
  display: none;
  position: absolute;
}

.video-ad-container.active {
  display: block;
}