In reply to Alfonso.

WordPress uses a variable $content_width since ages, which is also used to generate correct embed size/ratios etc.

https://codex.wordpress.org/Content_Width
https://developer.wordpress.com/themes/content-width/

Gutenberg does not seem to support this, so every theme needs to be updated especially for Gutenberg?

And thanks for your link, @Glenn Martin, question: Will these classnames remain like this also in Gutenberg final release? There are mixed answers of developers about the part "gutenberg" in code and classes.