Implement value property on <select>.

components/script/dom/htmloptionelement.rs

@@ -64,6 +64,10 @@ impl HTMLOptionElement {
         self.selectedness.set(selected);
     }
 
+    pub fn set_dirtiness(&self, dirtiness: bool) {
+        self.dirtiness.set(dirtiness);
+    }
+
     fn pick_if_selected_and_reset(&self) {
         if let Some(select) = self.upcast::<Node>().ancestors()
                 .filter_map(Root::downcast::<HTMLSelectElement>)

components/script/dom/htmlselectelement.rs

@@ -258,6 +258,37 @@ impl HTMLSelectElementMethods for HTMLSelectElement {
     fn Remove(&self) {
         self.upcast::<Element>().Remove()
     }
+
+    // https://html.spec.whatwg.org/multipage/#dom-select-value
+    fn Value(&self) -> DOMString {
+        self.upcast::<Node>()
+            .traverse_preorder()
+            .filter_map(Root::downcast::<HTMLOptionElement>)
+            .filter(|opt_elem| opt_elem.Selected())
+            .map(|opt_elem| opt_elem.Value())
+            .next()
+            .unwrap_or_default()
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-select-value
+    fn SetValue(&self, value: DOMString) {
+        let mut opt_iter = self.upcast::<Node>()
+                               .traverse_preorder()
+                               .filter_map(Root::downcast::<HTMLOptionElement>);
+        // Reset until we find an <option> with a matching value
+        for opt in opt_iter.by_ref() {
+            if opt.Value() == value {
+                opt.set_selectedness(true);
+                opt.set_dirtiness(true);
+                break;
+            }
+            opt.set_selectedness(false);
+        }
+        // Reset remaining <option> elements
+        for opt in opt_iter {
+            opt.set_selectedness(false);
+        }
+    }
 }
 
 impl VirtualMethods for HTMLSelectElement {

components/script/dom/webidls/HTMLSelectElement.webidl

@@ -26,7 +26,7 @@ interface HTMLSelectElement : HTMLElement {
 
   //readonly attribute HTMLCollection selectedOptions;
   //         attribute long selectedIndex;
-  //         attribute DOMString value;
+  attribute DOMString value;
 
   //readonly attribute boolean willValidate;
   readonly attribute ValidityState validity;