What voltage are the GPIO pins?

I've tried looking around for it, incl. https://nurdspace.nl/ESP8266, but I can't quite see where the voltage of the GPIO pins is specified. Does anyone know?
Thanks
Thanks
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martinayotte wrote:It is a 3.3V nominal device, so output voltage is quite the same. Specs says 0.8 x VDD for minimal Voh with a Imax of 12mA.