Allow empty string to be passed to formAction (#26379)

We disallow empty strings for `href` and `src` since they're common
mistakes that end up loading the current page as a preload, image or
link. We also disallow it for `action`. You have to pass `null` which is
the same.

However, for `formAction` passing `null` is not the same as passing
empty string. Passing empty string overrides the form's action to be the
current page even if the form's action was set to something else.
There's no easy way to express the same thing `#` show up in the user
visible URLs and `?` clears the search params.

Since this is also not a common mistake, we can just allow this.

packages/react-dom-bindings/src/shared/DOMProperty.js

@@ -636,7 +636,19 @@ properties[xlinkHref] = new PropertyInfoRecord(
   false, // removeEmptyString
 );
 
-['src', 'href', 'action', 'formAction'].forEach(attributeName => {
+const formAction = 'formAction';
+// $FlowFixMe[invalid-constructor] Flow no longer supports calling new on functions
+properties[formAction] = new PropertyInfoRecord(
+  'formAction',
+  STRING,
+  false, // mustUseProperty
+  'formaction', // attributeName
+  null, // attributeNamespace
+  true, // sanitizeURL
+  false, // removeEmptyString
+);
+
+['src', 'href', 'action'].forEach(attributeName => {
   // $FlowFixMe[invalid-constructor] Flow no longer supports calling new on functions
   properties[attributeName] = new PropertyInfoRecord(
     attributeName,

packages/react-dom/src/__tests__/ReactDOMComponent-test.js

@@ -533,29 +533,17 @@ describe('ReactDOMComponent', () => {
         expect(node.hasAttribute('action')).toBe(false);
       });
 
-      it('should not add an empty formAction attribute', () => {
+      it('allows empty string of a formAction to override the default of a parent', () => {
         const container = document.createElement('div');
-        expect(() =>
-          ReactDOM.render(<button formAction="" />, container),
-        ).toErrorDev(
-          'An empty string ("") was passed to the formAction attribute. ' +
-            'To fix this, either do not render the element at all ' +
-            'or pass null to formAction instead of an empty string.',
+        ReactDOM.render(
+          <form action="hello">
+            <button formAction="" />,
+          </form>,
+          container,
         );
-        const node = container.firstChild;
-        expect(node.hasAttribute('formAction')).toBe(false);
-
-        ReactDOM.render(<button formAction="abc" />, container);
-        expect(node.hasAttribute('formAction')).toBe(true);
-
-        expect(() =>
-          ReactDOM.render(<button formAction="" />, container),
-        ).toErrorDev(
-          'An empty string ("") was passed to the formAction attribute. ' +
-            'To fix this, either do not render the element at all ' +
-            'or pass null to formAction instead of an empty string.',
-        );
-        expect(node.hasAttribute('formAction')).toBe(false);
+        const node = container.firstChild.firstChild;
+        expect(node.hasAttribute('formaction')).toBe(true);
+        expect(node.getAttribute('formaction')).toBe('');
       });
 
       it('should not filter attributes for custom elements', () => {

packages/shared/ReactFeatureFlags.js

@@ -172,7 +172,7 @@ export const disableInputAttributeSyncing = false;
 // Filter certain DOM attributes (e.g. src, href) if their values are empty
 // strings. This prevents e.g. <img src=""> from making an unnecessary HTTP
 // request for certain browsers.
-export const enableFilterEmptyStringAttributesDOM = false;
+export const enableFilterEmptyStringAttributesDOM = __EXPERIMENTAL__;
 
 // Changes the behavior for rendering custom elements in both server rendering
 // and client rendering, mostly to allow JSX attributes to apply to the custom