They offer a mix of audio-only, video-only, and video+audio. Here is a list for a particular video (great music btw)

    > youtube-dl -F https://www.youtube.com/watch?v=GMiaVo_x1EY
    [youtube] Extracting URL: https://www.youtube.com/watch?v=GMiaVo_x1EY
    [youtube] GMiaVo_x1EY: Downloading webpage
    [youtube] GMiaVo_x1EY: Downloading android player API JSON
    [info] Available formats for GMiaVo_x1EY:
    ID  EXT   RESOLUTION FPS CH │   FILESIZE  TBR PROTO │ VCODEC       VBR ACODEC      ABR ASR MORE INFO
    ──────────────────────────────────────────────────────────────────────────────────────────────────────────────
    sb2 mhtml 48x27        0    │                 mhtml │ images                               storyboard
    sb1 mhtml 80x45        0    │                 mhtml │ images                               storyboard
    sb0 mhtml 160x90       0    │                 mhtml │ images                               storyboard
    599 m4a   audio only      2 │   13.42MiB  31k dash  │ audio only       mp4a.40.5   31k 22k ultralow, m4a_dash
    600 webm  audio only      2 │   16.34MiB  37k dash  │ audio only       opus        37k 48k ultralow, webm_dash
    139 m4a   audio only      2 │   21.27MiB  49k dash  │ audio only       mp4a.40.5   49k 22k low, m4a_dash
    249 webm  audio only      2 │   23.83MiB  55k dash  │ audio only       opus        55k 48k low, webm_dash
    250 webm  audio only      2 │   30.99MiB  71k dash  │ audio only       opus        71k 48k low, webm_dash
    140 m4a   audio only      2 │   56.45MiB 129k dash  │ audio only       mp4a.40.2  129k 44k medium, m4a_dash
    251 webm  audio only      2 │   65.21MiB 150k dash  │ audio only       opus       150k 48k medium, webm_dash
    17  3gp   176x144      6  1 │   21.50MiB  49k https │ mp4v.20.3    49k mp4a.40.2    0k 22k 144p
    597 mp4   256x144     13    │    4.97MiB  11k dash  │ avc1.4d400b  11k video only          144p, mp4_dash
    598 webm  256x144     13    │    6.97MiB  16k dash  │ vp9          16k video only          144p, webm_dash
    160 mp4   256x144     25    │    5.51MiB  13k dash  │ avc1.4d400c  13k video only          144p, mp4_dash
    278 webm  256x144     25    │   15.44MiB  35k dash  │ vp9          35k video only          144p, webm_dash
    133 mp4   426x240     25    │    9.53MiB  22k dash  │ avc1.4d4015  22k video only          240p, mp4_dash
    242 webm  426x240     25    │   28.68MiB  66k dash  │ vp9          66k video only          240p, webm_dash
    134 mp4   640x360     25    │   15.47MiB  35k dash  │ avc1.4d401e  35k video only          360p, mp4_dash
    18  mp4   640x360     25  2 │ ~ 73.46MiB 165k https │ avc1.42001E 165k mp4a.40.2    0k 44k 360p
    243 webm  640x360     25    │   55.53MiB 127k dash  │ vp9         127k video only          360p, webm_dash
    135 mp4   854x480     25    │   22.52MiB  52k dash  │ avc1.4d401e  52k video only          480p, mp4_dash
    244 webm  854x480     25    │   89.33MiB 205k dash  │ vp9         205k video only          480p, webm_dash
    22  mp4   1280x720    25  2 │ ~ 90.62MiB 203k https │ avc1.64001F 203k mp4a.40.2    0k 44k 720p
    136 mp4   1280x720    25    │   32.23MiB  74k dash  │ avc1.4d401f  74k video only          720p, mp4_dash
    247 webm  1280x720    25    │  166.48MiB 382k dash  │ vp9         382k video only          720p, webm_dash
    137 mp4   1920x1080   25    │  106.40MiB 244k dash  │ avc1.640028 244k video only          1080p, mp4_dash
    248 webm  1920x1080   25    │  271.78MiB 623k dash  │ vp9         623k video only          1080p, webm_dash