.fullscreen {
  background-color: #000;
  width: 100%;
  height: 100%;
}

.asciinema-player-wrapper {
  text-align: center;
  outline: none;

  &:fullscreen {
    .fullscreen;
  }

  &:-webkit-full-screen {
    .fullscreen;
  }

  &:-moz-full-screen {
    .fullscreen;
  }

  .asciinema-player {
    text-align: left;
    display: inline-block;
    padding: 0px;
    position: relative;
    box-sizing: content-box;
    -moz-box-sizing: content-box;
    -webkit-box-sizing: content-box;
    overflow: hidden;
    max-width: 100%;
  }
}

.fullscreen() {
  background-color: #000;
  width: 100%;
  height: 100%;
}

.asciinema-player-wrapper {
  text-align: center;
  outline: none;

  &:fullscreen {
    .fullscreen;
  }

  &:-webkit-full-screen {
    .fullscreen;
  }

  &:-moz-full-screen {
    .fullscreen;
  }

  .asciinema-player {
    text-align: left;
    display: inline-block;
    padding: 0px;
    position: relative;
    box-sizing: content-box;
    -moz-box-sizing: content-box;
    -webkit-box-sizing: content-box;
    overflow: hidden;
    max-width: 100%;
  }
}

Make .fullscreen only a mixin, not a real class
Make .fullscreen only a mixin, not a real class
