WPT details: Web platform tests will be added to ensure consistent behaviour when the disallowed comma is encountered by the parser.
This deprecation removes the ability to use "," as a separator between items in a permissions policy declaration (feature policy declaration) in an iframe tag. Developers should switch to ";" as the correct separator.
Chrome currently recognizes a non-standard "," as a separator between items in an iframe "allow" attribute. This behaviour is an accidental side effect of the way that we parse the attribute using the same parser as the 'Feature-Policy' header. (Commas were allowed by the header parser to support concatenating multiple headers together, but the attribute was always specced to contain a single serialized policy.)