USB charging, part 1: requirements

And despite all this well-written documentation, most devices out in the wild (and thus most chargers and so-forth) that aren't using USB-C ports simply wire up the data pins to specific voltages to advertise what amperage they support and the devices suck down power if that's high enough.

Thank Apple for simply implementing something years (a decade really) before any real (and quite complicated) standard came out, so that's become the de-jure standard even if it's not officially any kind of standard. And that de-jure standard is what those $8 wall-plug adapters your local membership-store has on the shelf, not ones that follow the full USB charger spec.