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

Handle properly alternate stylesheet

<!-- Please describe your changes on the following line: -->
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 ?
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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components/script/dom/htmllinkelement.rs

@@ -99,6 +99,17 @@ impl HTMLLinkElement {
             })
         })
     }
+
+    pub fn is_alternate(&self) -> bool {
+        let rel = get_attr(self.upcast(), &local_name!("rel"));
+        match rel {
+            Some(ref value) => {
+                value.split(HTML_SPACE_CHARACTERS)
+                    .any(|s| s.eq_ignore_ascii_case("alternate"))
+            },
+            None => false,
+        }
+    }
 }
 
 fn get_attr(element: &Element, local_name: &LocalName) -> Option<String> {
@@ -112,17 +123,8 @@ fn get_attr(element: &Element, local_name: &LocalName) -> Option<String> {
 fn string_is_stylesheet(value: &Option<String>) -> bool {
     match *value {
         Some(ref value) => {
-            let mut found_stylesheet = false;
-            for s in value.split(HTML_SPACE_CHARACTERS).into_iter() {
-                if s.eq_ignore_ascii_case("alternate") {
-                    return false;
-                }
-
-                if s.eq_ignore_ascii_case("stylesheet") {
-                    found_stylesheet = true;
-                }
-            }
-            found_stylesheet
+            value.split(HTML_SPACE_CHARACTERS)
+                .any(|s| s.eq_ignore_ascii_case("stylesheet"))
         },
         None => false,
     }

components/script/stylesheet_loader.rs

@@ -138,6 +138,9 @@ impl FetchResponseListener for StylesheetContext {
                                                         Some(&loader),
                                                         win.css_error_reporter(),
                                                         ParserContextExtraData::default()));
+                    if elem.downcast::<HTMLLinkElement>().unwrap().is_alternate() {
+                        sheet.set_disabled(true);
+                    }
                     elem.downcast::<HTMLLinkElement>()
                         .unwrap()
                         .set_stylesheet(sheet.clone());