Feature Description

On my recent blog post just noticed that my videos are as wide as the content width, but the poster image is still the full size URL. This could be improved by using a more appropriate image size. The video[poster] attribute doesn't support srcset or anything, so this needs to be done differently.

We could:

1. Detect width of the video
2. Check the poster URL and get the attachment ID from that URL
3. Get a smaller sized version of that image and use that as the poster instead.

@westonruter Something for Optimization Detective?

@joemcgill @mukeshpanchal27 Something that could perhaps also be done with the thing you're looking into where we take a block's layout into account for image sizes?