This is definitely either a bug or a change in the default behavior of Webflow’s video component. I started seeing this on a new site I’m working on, double-checked previous sites and it wasn’t there. Webflow’s video component uses Embedly to embed videos (a third party tool that just takes your video URL and standardizes the embed behavior regardless of platform). Previously the Embedly code that was generated from the video component added in a frameborder=“0” attribute to the resulting
Right now the only fixes appear to be those above - use the HTML Embed component manually, or add the style code @Stan provided. I’ve opened a ticket with support, so hopefully they can fix the issue.