In this part I'll talk more in-depth about video playback with the Acer C720.
The Acer C720 plays back all YouTube videos without a problem, I've not had any problem playing back Standard Definition, 720P HD, 1080P HD & even 4K HD. I really can't see the point of why anyone would want to playback a 4K video on an 11.6" Ultrabook, but out of curiosity when I saw the 4K option on a YouTube I had to try it out to see how well the Acer C720 would handle it.
Locally stored videos
The Acer C720 can handle locally stored or streamed over USB from an external HDD all video formats that you can throw at it ether natively or via a third-party video player, the only slight problem I've come across was when I tried to playback an episode of Game Of Thrones in .mkv format, where the video played-back but it couldn't playback the audio for some reason, and so far I've not found a way to playback .mkv encoded videos, but at least a Chromebook can be used as a portable media player & the inclusion of a full-sized HDMI port on the Acer C720 means that it can be hooked up to a TV and be used as an inexpensive media player even though the 16GB of SSD storage is a bit limiting.
I've not really tried using streaming video services like Netflix on the C720 yet but it's possible to store videos on Google Drive and stream them to the C720 but the free 100GB of Google Drive for 2 years combined with the free as standard 25GB of Google Drive storage space might be a bit of a limiting factor due to Google Drive being used to the rest of your data, and a subscription to a higher storage plan might be wise but once you go past the 100GB plan they start to get on the high price a month or year.
The Acer C720 is a capable machine for video playback, but you're going to have to think hard about whether you're going to store your videos on the limited internal storage or whether you're going to use an external HDD or SD Card.
If you want to stream videos I'd seriously consider using a service like Netflix over using your Google Drive storage as using Google Drive could become an expensive option considering that you also store your pictures and Documents on Google Drive.
Google could do with adding a video Upload feature to Play Movies & TV like they've got on Play Music, it'd allow Chromebook users along with Android users as they'd be able to watch their video Uploads on any device that supports the Play Movies & TV app along with any computer running Chrome that supports the use of Chrome web apps.