I'm noticing that all sites are getting this message in console :
[Deprecation] Styling master document from stylesheets defined in HTML Imports is deprecated, and is planned to be removed in M65, around March 2018. Please refer to the link for possible migration paths.
[Obsolete] The style sheet style document defined in HTML Imports is obsolete and is intended to be removed in M65, around March 2018. Please refer to the link for possible migration paths.
What would that be?