Add option to define the width of the inserted video (mp4)

@favorinus, Please help the community help you…

How you can help the community is well laid out in sticky post Welcome to the Grav CMS Community Forums

Please update you post.

Btw. feature request should be made in the Github repository of either Grav, theme, or plugin.

But before creating a feature request, take a look at the docs and learn how attributes can be added when adding image/video in Markdown, how custom css can be added to a theme, etc.