AFAIK chromecast and other HDMI dongles have secondary USB port to get power.

According to this Q&A, about 55-150 mA is available on a 5V line: http://www.hdmi.org/learningcenter/kb.aspx?c=13#42

Yep - the chromecast pulls too much juice (up to 1A) for the HDMI power spec.

1A would be the sweet spot for future HDMI specs.

That's on source devices, which in this case would be the Pi Zero. AFAIK, sink devices like a TV do not supply power.

I somehow managed to miss the sentence entirely where this is explained (it is used for reading EDID from a connected display). Thanks for the correction!

Oh I didn't know it was in spec. Also, it's just in range for a pi zero. How convenient.

The USB-powered XBox controller is going to need another 300 mA.

I also don't think the pi zero has a powerable HDMI adaptor.

I had a pi zero hdmi dongle in mind without a supplementary usb cable.

An on-board battery or capacitor could help to smooth out brief high power tasks, but I don't think it will be able to sustain video decoding & playback at a constant supply of 150mA. I'm really excited for that to be a possibility though, so I hope I'm wrong.

Of course, some TVs might dump a lot more current over the connection, just like what was common with USB 2.0 going well over 500mA.

It's technically possible, but supported on so few devices it's not worth the extra circuitry, I assume.

Chromecast requires external power. Google's advertising of it is misleading.

I've never seen any Chromecast advertising that implies it only gets power over HDMI. All of the media on Google's Chromecast site show either both HDMI/Aux and USB plugged in or nothing plugged in. Pretty much any TV now has a USB port whether it be for media playback or for updating the firmware of the TV on older ones so its not a huge deal to have to plug it.

Even the first release of the device clearly mentioned it needing USB power. https://www.youtube.com/watch?v=YRu18f2GRQo&t=41m22s

All of the screenshots I saw before purchasing the original device clearly showed it plugged into a TV with no power cord attached. It looks like they may have fixed it in the newer marketing.

Edit: I'm an idiot. You're right, I must have been thinking of CEC.

Nah. They rely on the fact that most modern TVs have a USB port near the HDMI ports. That usually provides enough juice.

Meaning power over HDMI is only provided when the TV supports MHL which most TVs don't support.

I know the rpi2 gets power over hdmi, not sure about the zero though