Auto merge of #14911 - charlesvdv:alternate-stylesheet, r=cbrewster

Handle properly alternate stylesheet

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Alternate stylesheet are now fetched and then desactivated instead of ignoring them.

I don't know if tests are required to check if the stylesheet is correctly loaded and disabled ?
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14881 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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tests/wpt/web-platform-tests/html/semantics/links/linktypes/alternate-import.css

@@ -0,0 +1,3 @@
+body {
+    background-color: black;
+}

tests/wpt/web-platform-tests/html/semantics/links/linktypes/alternate.css

@@ -1,3 +1,5 @@
+@import url("alternate-import.css");
+
 div {
     background-color: red;
 }