Restricting resolution
```plaintext
youtube-dl -f best[height=1920] $URL # restrict to <= 1080p
```
Specifying format
```plaintext
youtube-dl -f best[ext=mp4] $URL # mp4 files
```
Combining specifications
```plaintext
youtube-dl -f best[ext=mp4,height=1920] $URL # restrict to <= 1080p, mp4 files
```