Video blocks can cause validation errors in AMP Stories

[westonruter]

When adding a video block to an AMP Story page and hitting Save an AMP validation error appears:

This is due to the video not having:

• autoplay
• poster

When supplying these the validation error goes away:

When a video is shown in an AMP Stories context, the autoplay toggle should be enabled by default and either disabled or hidden entirely.

Also, the playsinline toggle needs to be removed in the stories context since it is not allowed:

Additionally, since we don't currently automatically extract a poster image from a video (see #903) there should be a warning displayed with either the poster field to indicate it is required.

[swissspidy]

To verify:

• When a video is shown in an AMP Stories context, the autoplay toggle should be enabled by default
• The "plays inline" option defaults to false
• When inserting a video, and the video has a featured image set, the featured image is automatically used as a poster image

[westonruter]

• When a video is shown in an AMP Stories context, the autoplay toggle should be enabled by default
• The "plays inline" option defaults to false

Note that the “autoplay” and “plays inline” options actually shouldn't be displayed at all:

The video should automatically be autoplaying when you preview on the frontend.

[csossi]

On selecting a video from media Library and returning to editor, the controls that should be hidden appear (Weston's note above):

Note that if I remove focus from video (click somewhere else) and THEN click on the video, THEN the controls are removed:

[swissspidy]

Thanks for testing! Given that we have #2283 for this specific issue with the hidden controls and focus, I am inclined to move this one to "Ready for merge" and continue testing on #2283 once that issue has been fixed.