Heartbeat Pulse beats your word to a resting heart rate while an ECG line traces itself across the frame behind it, spiking in time with each pulse.
hb-beat uses a double-thump envelope rather than a sine — the lub-dub of a real heartbeat, which is why it reads as a pulse instead of a throb. hb-trace draws the ECG with stroke-dashoffset in sync, and feGaussianBlur with feMerge gives the trace its monitor glow.
It is a stylised trace with the recognisable spike shape, timed to the beat. It is designed to read instantly, not to be clinically correct.