Handle setting selectionStart to be > selectionEnd

components/script/dom/textcontrol.rs

@@ -21,7 +21,16 @@ pub trait TextControl: DerivedFrom<EventTarget> + DerivedFrom<Node> {
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectionstart
     fn set_dom_selection_start(&self, start: u32) {
-        self.set_selection_range(start, self.dom_selection_end(), self.selection_direction());
+        // Step 2
+        let mut end = self.dom_selection_end();
+
+        // Step 3
+        if end < start {
+            end = start;
+        }
+
+        // Step 4
+        self.set_selection_range(start, end, self.selection_direction());
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-textarea/input-selectionend

tests/wpt/metadata/html/semantics/forms/textfieldselection/selection-start-end.html.ini

@@ -1,5 +0,0 @@
-[selection-start-end.html]
-  type: testharness
-  [Setting selectionStart to a value larger than selectionEnd should increase selectionEnd]
-    expected: FAIL
-