document behavior when no format options are specified

[jab]

https://github.com/rg3/youtube-dl#video-format-options does a great job explaining the behavior of the various format options. But what does youtube-dl choose if no format options are specified? Apologies if I missed this, and thanks for the great work. Awesome project!

[phihag]

As with all other options, the implicit default is the best option, where best is what (we think) the majority of users would prefer. In the case of the format, that's the highest video quality possible. (We're still working on re-enabling 1080p with default DASH merging, but the intention is to make that the default once it's implemented)

We reserve the right to switch the behavior though, although best quality will remain a guiding principle. For example, when a user passes in --extract-audio, we currently download the best audio-only format in some cases.

I've added a note saying that youtube-dl picks the best format by default.

[jab]

Thanks @phihag, that clarifies things, and I'm glad to hear youtube-dl defaults to choosing the highest quality version available. sindresorhus/quick-look-plugins#12 (comment) is what prompted me to open this issue, in case you wanted to take a peek.