[3] MP4 は、動画形式の1つです。

[1] ニコニコ動画のmp4(H.264)/flvの動画の音声を劣化無しで抽出する。 - NON ( 版)

[2] ( 版)

Representations with a value of the @mimeType attribute starting with a string other than

“xxx/mp4” where xxx is one of “video”, “audio”, “application” or “text” may be ignored.

Additional profile or codec specific parameters may be added to the end of the value of the

@mimeType attribute

[4] New WebKit Features in Safari 11.1 | WebKit ()

Animated image formats are very popular, but they easily become large, bandwidth intensive file sizes. To address the performance impact, WebKit in Safari now supports loading H.264 encoded MP4 video with an HTML <img> tag. This allows content authors to replace animated GIF files that are much larger than H.264 video files and require more processing power to display. Beyond the performance gains, this change also allows web content authors to use videos as a CSS background-image.

[6] >>5