.heartbeat-led {
    display: inline-block;
    width: 8px;
    height: 8px;
    margin-left: 0;
    margin-right: 10px;
    border-radius: 50%;
    background: white;
    opacity: 0.15;           /* stato di riposo: appena visibile, non sparisce mai */
    vertical-align: middle;
}


@keyframes heartbeat-pulse {
    0%     { box-shadow: 0 0 8px white; opacity: 1; }
    18.18% { box-shadow: none; opacity: 0.15; }
    81.82% { box-shadow: 0 0 12px white; opacity: 1; }
    100%   { box-shadow: none; opacity: 0.15; }
}

.heartbeat-led.is-beating {
    animation: heartbeat-pulse 385ms steps(1) 1;
}