Can you elaborate on why this should be included and link to some source about the solution?

Having max-width: 100% on media elements like img and video ensures that they don't overflow their container (for modern responsive web development). This is an opinionated thing so maybe it doesn't belong in modern-normalize but I can't think of a situation where you wouldn't want to prevent overflowing by default?
https://developer.mozilla.org/en-US/docs/Web/CSS/max-width

I added it to e811a7e instead. Thanks for the suggestion.