Tweak validation message, add html support

The old message made no sense if you had a > div > a or similar. I'm clearly feeling sloppy today.

src/browser/ui/__tests__/ReactDOMComponent-test.js

@@ -668,7 +668,8 @@ describe('ReactDOMComponent', function() {
 
       expect(console.error.calls.length).toBe(1);
       expect(console.error.calls[0].args[0]).toBe(
-        'Warning: validateDOMNesting(...): <div> cannot contain a <tr> node.'
+        'Warning: validateDOMNesting(...): <tr> cannot appear as a child of ' +
+        '<div> in this context (div > div).'
       );
     });
 
@@ -679,7 +680,8 @@ describe('ReactDOMComponent', function() {
 
       expect(console.error.calls.length).toBe(1);
       expect(console.error.calls[0].args[0]).toBe(
-        'Warning: validateDOMNesting(...): <p> cannot contain a <tr> node.'
+        'Warning: validateDOMNesting(...): <tr> cannot appear as a child of ' +
+        '<p> in this context (p).'
       );
     });
 
@@ -694,9 +696,10 @@ describe('ReactDOMComponent', function() {
 
       expect(console.error.calls.length).toBe(1);
       expect(console.error.calls[0].args[0]).toBe(
-        'Warning: validateDOMNesting(...): <table> cannot contain a <tr> ' +
-        'node. Add a <tbody> to your code to match the DOM tree generated by ' +
-        'the browser. Check the render method of `Foo`.'
+        'Warning: validateDOMNesting(...): <tr> cannot appear as a child of ' +
+        '<table> in this context (div > table). Add a <tbody> to your code ' +
+        'to match the DOM tree generated by the browser. Check the render ' +
+        'method of `Foo`.'
       );
     });
   });

src/browser/validateDOMNesting.js

@@ -176,6 +176,10 @@ if (__DEV__) {
           tag === 'noscript' || tag === 'noframes' || tag === 'style' ||
           tag === 'script' || tag === 'template'
         );
+
+      // https://html.spec.whatwg.org/multipage/semantics.html#the-html-element
+      case 'html':
+        return tag === 'head' || tag === 'body';
     }
 
     // Probably in the "in body" parsing mode, so we outlaw only tag combos
@@ -292,9 +296,11 @@ if (__DEV__) {
 
       warning(
         false,
-        'validateDOMNesting(...): <%s> cannot contain a <%s> node.%s',
-        parentTag,
+        'validateDOMNesting(...): <%s> cannot appear as a child of <%s> ' +
+        'in this context (%s).%s',
         childTag,
+        parentTag,
+        parentStack.join(' > '),
         info
       );
     }