How is text/event-stream checked?

[domenic]

Spinning off from https://github.com/whatwg/html/pull/7553/files#r808234276

The spec currently says

HTTP 200 OK responses with a [Content-Type](https://html.spec.whatwg.org/commit-snapshots/b3a49bcf51d5cf39f6233b752f631940a44677ab/#content-type) header specifying the type [text/event-stream](https://html.spec.whatwg.org/commit-snapshots/b3a49bcf51d5cf39f6233b752f631940a44677ab/#text/event-stream), ignoring any MIME type parameters, must be processed line by line as described below.

HTTP 200 OK responses that have a Content-Type specifying an unsupported type, or that have no Content-Type at all, must cause the user agent to fail the connection.

The exact phrasing will move around a bit as part of #7553 but we're decided not to try to make it more rigorous yet. So:

What about text/EVENT-stream? What about multiple Content-Type headers? What about whitespace?

The proper architecture here is to use https://fetch.spec.whatwg.org/#concept-header-extract-mime-type and compare the essences. We should do that. But we should also write web platform tests for the weird cases, as part of that work.