Are you really asserting that a CSS selector is a private API? This is either a really wild misunderstanding about the difference between CSS and API, or somehow I totally misread your post. But I did re-read a few times and that seems to be the claim?
The CSS isn't the issue, it's that the CSS can only be enabled using private platform code, which no app will get approval for.
Did you click my link or read the article? The private API is the WKWebView property.