Since the release of first 12.9″ iPad Pro in 2015, I’ve done all of my web development work on an iPad Pro.
Ed. Note: As of March 2019, I’m currently using a 64GB Space Gray Wifi-only 12.9″ iPad Pro in a charcoal gray Smart Folio, connected via Bluetooth to a Magic Keyboard. Previously, I was using a 64 GB Space Gray Wifi/Cellular 10.5″ iPad Pro with a Smart Keyboard attached.
Not only is this site coded entirely with an iPad, I also use it at work to build the ON1 site.
As you’re reading this, keep in mind that this is what works for me. Your mileage may vary depending on your setup.