Whether an airline seat-back USB port works safely and effectively with a tablet usually comes down to matching connector type and whether both sides support a shared negotiation standard — not raw wattage compatibility alone.
When standards match, expect normal or reduced-speed charging with no risk to the device. When they don’t match at the protocol level (for example, a magnetic charger on a device without the magnetic ring, or an inverter introducing power quality issues), charging may not start at all, or may run measurably slower and less efficiently.
Quick check
If the connector physically fits and both sides use an open standard (USB-PD, Qi/Qi2), it’s almost always safe to try — worst case is slow charging, not damage.