servo: Merge #13762 - Implement "reset algorithm" for `<select>` (from frewsxcv:select-reset); r=Manishearth

Source-Repo: https://github.com/servo/servo
Source-Revision: 28be83bd9a5b2adc136bfdf006ae5d285d0178ad

servo/components/script/dom/htmlformelement.rs

@@ -634,8 +634,7 @@ impl HTMLFormElement {
                 //    {}
                 //}
                 NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLSelectElement)) => {
-                    // Unimplemented
-                    {}
+                    child.downcast::<HTMLSelectElement>().unwrap().reset();
                 }
                 NodeTypeId::Element(ElementTypeId::HTMLElement(HTMLElementTypeId::HTMLTextAreaElement)) => {
                     child.downcast::<HTMLTextAreaElement>().unwrap().reset();

servo/components/script/dom/htmlselectelement.rs

@@ -84,6 +84,16 @@ impl HTMLSelectElement {
                            HTMLSelectElementBinding::Wrap)
     }
 
+    // https://html.spec.whatwg.org/multipage/#the-select-element:concept-form-reset-control
+    pub fn reset(&self) {
+        let node = self.upcast::<Node>();
+        for opt in node.traverse_preorder().filter_map(Root::downcast::<HTMLOptionElement>) {
+            opt.set_selectedness(opt.DefaultSelected());
+            opt.set_dirtiness(false);
+        }
+        self.ask_for_reset();
+    }
+
     // https://html.spec.whatwg.org/multipage/#ask-for-a-reset
     pub fn ask_for_reset(&self) {
         if self.Multiple() {