Disable auto fullscreen of YouTube embeds on iPhone

Add playsinline=1 paramerer to the embed url. Add ? or & before as appropriate; ? if the only paramerter, & to concatenate with other params.

Example:

<iframe
  src="https://www.youtube.com/v/VIDEO_ID?playsinline=1">
</iframe>

From YouTube iFrame Player API:

This parameter controls whether videos play inline or fullscreen in an HTML5 player on iOS. Valid values are: 0: This value causes fullscreen playback. This is currently the default value, though the default is subject to change. 1: This value causes inline playback for UIWebViews created with the allowsInlineMediaPlayback property set to TRUE.