include note on HTML5 custom elements

Author: Nicole Levy

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

@@ -1003,7 +1003,8 @@ describe('ReactDOMComponent', () => {
       }).toWarnDev(
         '<BR /> is using incorrect casing. ' +
           'Use PascalCase for React components, ' +
-          'or lowercase letters and dashes for HTML.',
+          'or lowercase letters and dashes for HTML ' +
+          'and HTML5 custom elements',
       );
       expect(returnedValue).not.toContain('</BR>');
     });
@@ -1019,7 +1020,8 @@ describe('ReactDOMComponent', () => {
       ).toWarnDev(
         '<IMG /> is using incorrect casing. ' +
           'Use PascalCase for React components, ' +
-          'or lowercase letters and dashes for HTML.',
+          'or lowercase letters and dashes for HTML ' +
+          'and HTML5 custom elements',
       );
     });
 
@@ -1069,7 +1071,8 @@ describe('ReactDOMComponent', () => {
         ).toWarnDev([
           '<hasOwnProperty /> is using incorrect casing. ' +
             'Use PascalCase for React components, ' +
-            'or lowercase letters and dashes for HTML.',
+            'or lowercase letters and dashes for HTML ' +
+            'and HTML5 custom elements',
           'The tag <hasOwnProperty> is unrecognized in this browser',
         ]);
       } finally {

packages/react-dom/src/__tests__/ReactServerRendering-test.internal.js

@@ -604,11 +604,13 @@ describe('ReactDOMServer', () => {
     ).toWarnDev([
       'Warning: <inPUT /> is using incorrect casing. ' +
         'Use PascalCase for React components, ' +
-        'or lowercase letters and dashes for HTML.',
+        'or lowercase letters and dashes for HTML ' +
+        'and HTML5 custom elements',
       // linearGradient doesn't warn
       'Warning: <iFrame /> is using incorrect casing. ' +
         'Use PascalCase for React components, ' +
-        'or lowercase letters and dashes for HTML.',
+        'or lowercase letters and dashes for HTML ' +
+        'and HTML5 custom elements',
     ]);
   });
 

packages/react-dom/src/client/ReactDOMFiberComponent.js

@@ -357,7 +357,8 @@ export function createElement(
         isCustomComponentTag || type === type.toLowerCase(),
         '<%s /> is using incorrect casing. ' +
           'Use PascalCase for React components, ' +
-          'or lowercase letters and dashes for HTML.',
+          'or lowercase letters and dashes for HTML ' +
+          'and HTML5 custom elements',
         type,
       );
     }

packages/react-dom/src/server/ReactPartialRenderer.js

@@ -957,7 +957,8 @@ class ReactDOMServerRenderer {
           tag === element.type,
           '<%s /> is using incorrect casing. ' +
             'Use PascalCase for React components, ' +
-            'or lowercase letters and dashes for HTML.',
+            'or lowercase letters and dashes for HTML ' +
+            'and HTML5 custom elements',
           element.type,
         );
       }