Improve <style> and <script> processing and conformance

* De-genericizes <style> and <link rel="stylesheet"> to only deal with
  CSS. Fixes #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 #2996.
* Adds pointers to #2997.
* Makes it clearer that parameters are not allowed in the content type
  value for script or style. Fixes #3022.