Auto merge of #15882 - ak1t0:clean-up-htmlimageelement, r=Ms2ger

Clean up HTMLImageElement::areas

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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 #15835 (github issue number if applicable).

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components/script/dom/htmlimageelement.rs

@@ -413,25 +413,18 @@ impl HTMLImageElement {
     }
     pub fn areas(&self) -> Option<Vec<Root<HTMLAreaElement>>> {
         let elem = self.upcast::<Element>();
-        let usemap_attr;
-        if elem.has_attribute(&LocalName::from("usemap")) {
-            usemap_attr = elem.get_string_attribute(&local_name!("usemap"));
-        } else {
-            return None;
+        let usemap_attr = match elem.get_attribute(&ns!(), &local_name!("usemap")) {
+            Some(attr) => attr,
+            None => return None,
+        };
+
+        let value = usemap_attr.value();
+        let (first, last) = value.split_at(1);
+
+        if first != "#" || last.len() == 0 {
+            return None
         }
 
-        let (first, last) = usemap_attr.split_at(1);
-
-        match first {
-            "#" => {},
-            _ => return None,
-        };
-
-        match last.len() {
-            0 => return None,
-            _ => {},
-        };
-
         let map = self.upcast::<Node>()
                       .following_siblings()
                       .filter_map(Root::downcast::<HTMLMapElement>)