html, body {
    margin: 0;
    padding: 0;
}
body {
    background: #f0f0f0;
}
.box {
    background: #e50020;
    color: #fff;
    font: bold 32px monospace;
    height: 100px;
    line-height: 100px;
    text-align: center;
    width: 100px;
}
.indicator {
    display: flex;
    justify-content: center;
    padding: 10px 0;
    position: fixed;
    top: 0;
    width: 100%;
}
.indicator-item {
    background: #ccc;
    border-radius: 50%;
    color: #fff;
    font: bold 16px monospace;
    height: 30px;
    line-height: 30px;
    margin: 0 10px;
    position: relative;
    text-align: center;
    width: 30px;
}
.indicator-item:after {
    /* background: rgba(130, 260, 0, 1); */
    background: rgba(0, 0, 0, 0.5);
    border-radius: 50%;
    content: "";
    display: block;
    height: 100%;
    left: 0;
    position: absolute;
    top: 0;
    width: 100%;
    z-index: -1;
}
.indicator-item.on {
    background: #a0e600;
}
.indicator-item.pulse:after {
    animation: pulse-animation 0.5s;
}

@keyframes pulse-animation {
  to {
    background: rgba(0, 0, 0, 0);
    transform: scale(1.5);
  }
}