DEFER or ASYNC allowed on a stylesheet include?

You could do this:

<link rel="stylesheet" href="/css/style.css" media="none" onload="if(media!=='all')media='all'" >

and create a noscript fallback

Defer and Async are specific attributes of the tag <script> https://developer.mozilla.org/en/docs/Web/HTML/Element/script

They will not work in other tags, because they don't exist there. A stylesheet is not a script that contains logic to be executed in parallel or after the loading. A stylesheet is a list of static styles that get applied atomically to html.