Looking at link-stylesheet[1], 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".

[1] https://html.spec.whatwg.org/multipage/semantics.html#link-type-stylesheet