Add styles for inline issues (or lint against them)

[tabatkins]

The WebIDL spec has an inline issue (the first paragraph of https://webidl.spec.whatwg.org/#idl-callback-functions). When it was a W3C spec this recieved appropriate issue styling, but it appears that the WHATWG stylesheets don't have styles for inline issues. This makes that paragraph look somewhat confusing.

We should either develop some styling for inline issues, or lint for inline issues to prevent them from creeping in, if we decide not to use them.

[annevk]

It would indeed be good to lint for this. I'm solving the case you identified in whatwg/webidl#1355 meanwhile.