Improve <style> and <script> processing and conformance

* De-genericizes <style> and <link rel="stylesheet"> to only deal with
  CSS. Fixes whatwg#2995.
* Makes type="" on <style> "obsolete but conforming", since it is always
  redundant.
* Makes type="(a JS MIME type)" on <script> obsolete but conforming as
  well. Previously we had a "should" requirement but had not recorded it
  in the centralized obsolete-but-conforming section that collects such
  requirements.
* Makes <style> operate on child text content. Fixes whatwg#2996.
* Replaces the conformance requirement (noted in the source as
  "temporary") prohibiting unmatched comment-like syntax inside <style>
  with a conformance requirement to be valid CSS.
* Adds pointers to whatwg#2997.
* Makes it clearer that parameters are not allowed in the content type
  value for script or style. Fixes whatwg#3022.