[dom] Link stylesheet's "appropriate times to obtain resource" doesn't cover all cases (#167)

Copying this one over from [Bugzilla](https://www.w3.org/Bugs/Public/show_bug.cgi?id=29078):

Looking at [link-stylesheet](https://html.spec.whatwg.org/multipage/semantics.html#link-type-stylesheet), I believe that the "appropriate times to obtain a resource" algorithm is not covering all cases.

* "When the external resource link is created on a link element that is already in a Document." does not include a conditional that would prevent obtaining the resource if `type` is not supported.
* Same for "When the external resource link's link element is inserted into a document."
* As for "When the type attribute of the link element of an external resource link that is already in a Document but was previously not obtained due to the type attribute specifying an unsupported type is set, removed, or changed." I believe that "set" is not possible (since if type wasn't set, the resource would have been obtained), and that "changed" should also include a condition such as "changed to a supported type".


---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/167