![2girls animated bow bubble_skirt closed_eyes crossed_arms eyeball floating furukawa_(yomawari) hat hat_bow hat_ribbon hata_no_kokoro head_tilt hill komeiji_koishi leaf long_hair long_sleeves mask mountain mp4 multiple_girls no_mouth no_nose pink_eyes pink_hair ribbon shirt short_hair sitting sitting_on_tree_stump skirt sleeves_past_wrists surreal third_eye touhou tree_stump white_hair yellow_bow yellow_ribbon yellow_shirt rating:s score:7 post #2611192](/cdn_image/preview/72/15/7215109c10e518058080cf951ddae4da.jpg)
MPEG-4 Part 14 or MP4 is a multi-purpose container format based on Apple's QuickTime format. It is an ISO standard (ISO/IEC 14496-14). It can be used for audio as well as video. It is mostly used to contain H.264 (MPEG-4 Part 10) compressed video with/and/or AAC (Advanced Audio Coding) or MP3 compressed audio. Modern browsers can directly stream MP4 files.
H.264 is the most used video format for video streaming while AAC is the successor to MP3, being more efficient than its predecessor. MP3 is still by far the most used audio format.
This tag implicates animated (learn more).