When paramedics arrive, the first things they ask are: what medicines, any allergies, any heart problems before? Most people can't remember under stress. Fill this in once and it's ready when you need it.
Your details never leave this device. There's no account and no server. Everything is saved in this browser on this phone only. Clearing your browser data deletes it, and so does the "Delete my card" button.
Tip: Add this site to your home screen so the card opens with no signal. Paramedics can also see it on the "For the paramedics" screen at the end of Emergency Mode. On iPhone and Android you can also fill in the built-in Medical ID or Emergency information, which shows on the lock screen.