diff --git a/packages/react-dom-bindings/src/client/DOMPropertyOperations.js b/packages/react-dom-bindings/src/client/DOMPropertyOperations.js
index f0649b97522b4..d89f40037ef2f 100644
--- a/packages/react-dom-bindings/src/client/DOMPropertyOperations.js
+++ b/packages/react-dom-bindings/src/client/DOMPropertyOperations.js
@@ -253,10 +253,6 @@ export function getValueForAttributeOnCustomComponent(
       }
       return expected === undefined ? undefined : null;
     }
-    if (enableCustomElementPropertySupport && name === 'className') {
-      // className is a special cased property on the server to render as an attribute.
-      name = 'class';
-    }
     const value = node.getAttribute(name);
 
     if (enableCustomElementPropertySupport) {
@@ -448,11 +444,7 @@ export function setValueForPropertyOnCustomComponent(
   name: string,
   value: mixed,
 ) {
-  if (
-    enableCustomElementPropertySupport &&
-    name[0] === 'o' &&
-    name[1] === 'n'
-  ) {
+  if (name[0] === 'o' && name[1] === 'n') {
     const useCapture = name.endsWith('Capture');
     const eventName = name.substr(2, useCapture ? name.length - 9 : undefined);
 
@@ -477,40 +469,16 @@ export function setValueForPropertyOnCustomComponent(
     }
   }
 
-  if (enableCustomElementPropertySupport && name in (node: any)) {
+  if (name in (node: any)) {
     (node: any)[name] = value;
     return;
   }
 
-  if (isAttributeNameSafe(name)) {
-    // shouldRemoveAttribute
-    if (value === null) {
-      node.removeAttribute(name);
-      return;
-    }
-    switch (typeof value) {
-      case 'undefined':
-      case 'function':
-      case 'symbol': // eslint-disable-line
-        node.removeAttribute(name);
-        return;
-      case 'boolean': {
-        if (enableCustomElementPropertySupport) {
-          if (value === true) {
-            node.setAttribute(name, '');
-            return;
-          }
-          node.removeAttribute(name);
-          return;
-        }
-      }
-    }
-    if (__DEV__) {
-      checkAttributeStringCoercion(value, name);
-    }
-    node.setAttribute(
-      name,
-      enableTrustedTypesIntegration ? (value: any) : '' + (value: any),
-    );
+  if (value === true) {
+    node.setAttribute(name, '');
+    return;
   }
+
+  // From here, it's the same as any attribute
+  setValueForAttribute(node, name, value);
 }
diff --git a/packages/react-dom-bindings/src/client/ReactDOMComponent.js b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
index 93686a54b8b7e..e587d84707189 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMComponent.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
@@ -15,7 +15,6 @@ import {
 } from '../events/EventRegistry';
 
 import {canUseDOM} from 'shared/ExecutionEnvironment';
-import hasOwnProperty from 'shared/hasOwnProperty';
 import {checkHtmlStringCoercion} from 'shared/CheckStringCoercion';
 
 import {
@@ -59,15 +58,13 @@ import {
 } from './CSSPropertyOperations';
 import {HTML_NAMESPACE, getIntrinsicNamespace} from './DOMNamespaces';
 import {getPropertyInfo} from '../shared/DOMProperty';
-import {DOCUMENT_NODE} from './HTMLNodeType';
-import isCustomComponent from '../shared/isCustomComponent';
+import isCustomElement from '../shared/isCustomElement';
 import possibleStandardNames from '../shared/possibleStandardNames';
 import {validateProperties as validateARIAProperties} from '../shared/ReactDOMInvalidARIAHook';
 import {validateProperties as validateInputProperties} from '../shared/ReactDOMNullInputValuePropHook';
 import {validateProperties as validateUnknownProperties} from '../shared/ReactDOMUnknownPropertyHook';
 
 import {
-  enableTrustedTypesIntegration,
   enableCustomElementPropertySupport,
   enableClientRenderFallbackOnTextMismatch,
   enableHostSingletons,
@@ -79,25 +76,8 @@ import {
 } from '../events/DOMPluginEventSystem';
 
 let didWarnInvalidHydration = false;
-let didWarnScriptTags = false;
-
-let warnedUnknownTags: {
-  [key: string]: boolean,
-};
 let canDiffStyleForHydrationWarning;
-
 if (__DEV__) {
-  warnedUnknownTags = {
-    // There are working polyfills for <dialog>. Let people use it.
-    dialog: true,
-    // Electron ships a custom <webview> tag to display external web content in
-    // an isolated frame and process.
-    // This tag is not present in non Electron environments such as JSDom which
-    // is often used for testing purposes.
-    // @see https://electronjs.org/docs/api/webview-tag
-    webview: true,
-  };
-
   // IE 11 parses & normalizes the style attribute as opposed to other
   // browsers. It adds spaces and sorts the properties in some
   // non-alphabetical order. Handling that would require sorting CSS
@@ -264,14 +244,6 @@ export function checkForUnmatchedText(
   }
 }
 
-export function getOwnerDocumentFromRootContainer(
-  rootContainerElement: Element | Document | DocumentFragment,
-): Document {
-  return rootContainerElement.nodeType === DOCUMENT_NODE
-    ? (rootContainerElement: any)
-    : rootContainerElement.ownerDocument;
-}
-
 function noop() {}
 
 export function trapClickOnNonInteractiveElement(node: HTMLElement) {
@@ -292,7 +264,7 @@ function setProp(
   tag: string,
   key: string,
   value: mixed,
-  isCustomComponentTag: boolean,
+  isCustomElementTag: boolean,
   props: any,
 ): void {
   switch (key) {
@@ -418,8 +390,16 @@ function setProp(
           warnForInvalidEventListener(key, value);
         }
       } else {
-        if (isCustomComponentTag) {
-          setValueForPropertyOnCustomComponent(domElement, key, value);
+        if (isCustomElementTag) {
+          if (enableCustomElementPropertySupport) {
+            setValueForPropertyOnCustomComponent(domElement, key, value);
+          } else {
+            if (typeof value === 'boolean') {
+              // Special case before the new flag is on
+              value = '' + (value: any);
+            }
+            setValueForAttribute(domElement, key, value);
+          }
         } else {
           if (
             // shouldIgnoreAttribute
@@ -443,136 +423,6 @@ function setProp(
   }
 }
 
-// creates a script element that won't execute
-export function createPotentiallyInlineScriptElement(
-  ownerDocument: Document,
-): Element {
-  // Create the script via .innerHTML so its "parser-inserted" flag is
-  // set to true and it does not execute
-  const div = ownerDocument.createElement('div');
-  if (__DEV__) {
-    if (enableTrustedTypesIntegration && !didWarnScriptTags) {
-      console.error(
-        'Encountered a script tag while rendering React component. ' +
-          'Scripts inside React components are never executed when rendering ' +
-          'on the client. Consider using template tag instead ' +
-          '(https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template).',
-      );
-      didWarnScriptTags = true;
-    }
-  }
-  div.innerHTML = '<script><' + '/script>'; // eslint-disable-line
-  // This is guaranteed to yield a script element.
-  const firstChild = ((div.firstChild: any): HTMLScriptElement);
-  const element = div.removeChild(firstChild);
-  return element;
-}
-
-export function createSelectElement(
-  props: Object,
-  ownerDocument: Document,
-): Element {
-  let element;
-  if (typeof props.is === 'string') {
-    element = ownerDocument.createElement('select', {is: props.is});
-  } else {
-    // Separate else branch instead of using `props.is || undefined` above because of a Firefox bug.
-    // See discussion in https://github.com/facebook/react/pull/6896
-    // and discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1276240
-    element = ownerDocument.createElement('select');
-  }
-  if (props.multiple) {
-    element.multiple = true;
-  } else if (props.size) {
-    // Setting a size greater than 1 causes a select to behave like `multiple=true`, where
-    // it is possible that no option is selected.
-    //
-    // This is only necessary when a select in "single selection mode".
-    element.size = props.size;
-  }
-  return element;
-}
-
-// Creates elements in the HTML namesapce
-export function createHTMLElement(
-  type: string,
-  props: Object,
-  ownerDocument: Document,
-): Element {
-  if (__DEV__) {
-    switch (type) {
-      case 'script':
-      case 'select':
-        console.error(
-          'createHTMLElement was called with a "%s" type. This type has special creation logic in React and should use the create function implemented specifically for it. This is a bug in React.',
-          type,
-        );
-        break;
-      case 'svg':
-      case 'math':
-        console.error(
-          'createHTMLElement was called with a "%s" type. This type must be created with Document.createElementNS which this method does not implement. This is a bug in React.',
-          type,
-        );
-    }
-  }
-
-  let isCustomComponentTag;
-
-  let element: Element;
-  if (__DEV__) {
-    isCustomComponentTag = isCustomComponent(type, props);
-    // Should this check be gated by parent namespace? Not sure we want to
-    // allow <SVG> or <mATH>.
-    if (!isCustomComponentTag && type !== type.toLowerCase()) {
-      console.error(
-        '<%s /> is using incorrect casing. ' +
-          'Use PascalCase for React components, ' +
-          'or lowercase for HTML elements.',
-        type,
-      );
-    }
-  }
-
-  if (typeof props.is === 'string') {
-    element = ownerDocument.createElement(type, {is: props.is});
-  } else {
-    // Separate else branch instead of using `props.is || undefined` above because of a Firefox bug.
-    // See discussion in https://github.com/facebook/react/pull/6896
-    // and discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1276240
-    element = ownerDocument.createElement(type);
-  }
-
-  if (__DEV__) {
-    if (
-      !isCustomComponentTag &&
-      // $FlowFixMe[method-unbinding]
-      Object.prototype.toString.call(element) ===
-        '[object HTMLUnknownElement]' &&
-      !hasOwnProperty.call(warnedUnknownTags, type)
-    ) {
-      warnedUnknownTags[type] = true;
-      console.error(
-        'The tag <%s> is unrecognized in this browser. ' +
-          'If you meant to render a React component, start its name with ' +
-          'an uppercase letter.',
-        type,
-      );
-    }
-  }
-
-  return element;
-}
-
-export function createTextNode(
-  text: string,
-  rootContainerElement: Element | Document | DocumentFragment,
-): Text {
-  return getOwnerDocumentFromRootContainer(rootContainerElement).createTextNode(
-    text,
-  );
-}
-
 export function setInitialProperties(
   domElement: Element,
   tag: string,
@@ -807,7 +657,6 @@ export function setInitialProperties(
           }
           // defaultChecked and defaultValue are ignored by setProp
           default: {
-            // TODO: If the `is` prop is specified, this should go through the isCustomComponentTag flow.
             setProp(domElement, tag, propKey, propValue, false, props);
           }
         }
@@ -816,7 +665,7 @@ export function setInitialProperties(
     }
   }
 
-  const isCustomComponentTag = isCustomComponent(tag, props);
+  const isCustomElementTag = isCustomElement(tag, props);
   for (const propKey in props) {
     if (!props.hasOwnProperty(propKey)) {
       continue;
@@ -825,7 +674,7 @@ export function setInitialProperties(
     if (propValue == null) {
       continue;
     }
-    setProp(domElement, tag, propKey, propValue, isCustomComponentTag, props);
+    setProp(domElement, tag, propKey, propValue, isCustomElementTag, props);
   }
 }
 
@@ -920,6 +769,13 @@ export function diffProperties(
           }
           break;
         }
+        case 'is':
+          if (__DEV__) {
+            console.error(
+              'Cannot update the "is" prop after it has been initialized.',
+            );
+          }
+        // eslint-disable-next-line no-fallthrough
         default: {
           (updatePayload = updatePayload || []).push(propKey, nextProp);
         }
@@ -1095,7 +951,6 @@ export function updateProperties(
           }
           // defaultChecked and defaultValue are ignored by setProp
           default: {
-            // TODO: If the `is` prop is specified, this should go through the isCustomComponentTag flow.
             setProp(domElement, tag, propKey, propValue, false, nextProps);
           }
         }
@@ -1104,21 +959,12 @@ export function updateProperties(
     }
   }
 
-  // TODO: Handle wasCustomComponentTag. Changing "is" isn't valid.
-  // const wasCustomComponentTag = isCustomComponent(tag, lastProps);
-  const isCustomComponentTag = isCustomComponent(tag, nextProps);
+  const isCustomElementTag = isCustomElement(tag, nextProps);
   // Apply the diff.
   for (let i = 0; i < updatePayload.length; i += 2) {
     const propKey = updatePayload[i];
     const propValue = updatePayload[i + 1];
-    setProp(
-      domElement,
-      tag,
-      propKey,
-      propValue,
-      isCustomComponentTag,
-      nextProps,
-    );
+    setProp(domElement, tag, propKey, propValue, isCustomElementTag, nextProps);
   }
 }
 
@@ -1218,6 +1064,21 @@ function diffHydratedCustomComponent(
           continue;
         }
       // eslint-disable-next-line no-fallthrough
+      case 'className':
+        if (enableCustomElementPropertySupport) {
+          // className is a special cased property on the server to render as an attribute.
+          extraAttributeNames.delete('class');
+          const serverValue = getValueForAttributeOnCustomComponent(
+            domElement,
+            'class',
+            nextProp,
+          );
+          if (nextProp !== serverValue) {
+            warnForPropDifference('className', serverValue, nextProp);
+          }
+          continue;
+        }
+      // eslint-disable-next-line no-fallthrough
       default: {
         let ownNamespaceDev = parentNamespaceDev;
         if (ownNamespaceDev === HTML_NAMESPACE) {
@@ -1504,7 +1365,7 @@ export function diffHydratedProperties(
           extraAttributeNames.add(attributes[i].name);
       }
     }
-    if (isCustomComponent(tag, props)) {
+    if (isCustomElement(tag, props)) {
       diffHydratedCustomComponent(
         domElement,
         tag,
diff --git a/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js b/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
index 5d2451b00ce10..253b2da401cc8 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
@@ -44,10 +44,6 @@ import {
 export {detachDeletedInstance};
 import {hasRole} from './DOMAccessibilityRoles';
 import {
-  createHTMLElement,
-  createPotentiallyInlineScriptElement,
-  createSelectElement,
-  createTextNode,
   setInitialProperties,
   diffProperties,
   updateProperties,
@@ -59,7 +55,6 @@ import {
   warnForDeletedHydratableText,
   warnForInsertedHydratedElement,
   warnForInsertedHydratedText,
-  getOwnerDocumentFromRootContainer,
 } from './ReactDOMComponent';
 import {getSelectionInformation, restoreSelection} from './ReactInputSelection';
 import setTextContent from './setTextContent';
@@ -90,6 +85,7 @@ import {
   enableScopeAPI,
   enableFloat,
   enableHostSingletons,
+  enableTrustedTypesIntegration,
 } from 'shared/ReactFeatureFlags';
 import {
   HostComponent,
@@ -119,6 +115,9 @@ export type Props = {
   left?: null | number,
   right?: null | number,
   top?: null | number,
+  is?: string,
+  size?: number,
+  multiple?: boolean,
   ...
 };
 type RawProps = {
@@ -182,6 +181,14 @@ let selectionInformation: null | SelectionInformation = null;
 
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoPersistence';
 
+function getOwnerDocumentFromRootContainer(
+  rootContainerElement: Element | Document | DocumentFragment,
+): Document {
+  return rootContainerElement.nodeType === DOCUMENT_NODE
+    ? (rootContainerElement: any)
+    : rootContainerElement.ownerDocument;
+}
+
 export function getRootHostContext(
   rootContainerInstance: Container,
 ): HostContext {
@@ -285,7 +292,8 @@ export function createHoistableInstance(
   const ownerDocument = getOwnerDocumentFromRootContainer(
     rootContainerInstance,
   );
-  const domElement: Instance = createHTMLElement(type, props, ownerDocument);
+
+  const domElement: Instance = ownerDocument.createElement(type);
   precacheFiberNode(internalInstanceHandle, domElement);
   updateFiberProps(domElement, props);
   setInitialProperties(domElement, type, props);
@@ -293,6 +301,20 @@ export function createHoistableInstance(
   return domElement;
 }
 
+let didWarnScriptTags = false;
+const warnedUnknownTags: {
+  [key: string]: boolean,
+} = {
+  // There are working polyfills for <dialog>. Let people use it.
+  dialog: true,
+  // Electron ships a custom <webview> tag to display external web content in
+  // an isolated frame and process.
+  // This tag is not present in non Electron environments such as JSDom which
+  // is often used for testing purposes.
+  // @see https://electronjs.org/docs/api/webview-tag
+  webview: true,
+};
+
 export function createInstance(
   type: string,
   props: Props,
@@ -334,20 +356,94 @@ export function createInstance(
       break;
     default:
       switch (type) {
-        case 'svg':
+        case 'svg': {
           domElement = ownerDocument.createElementNS(SVG_NAMESPACE, type);
           break;
-        case 'math':
+        }
+        case 'math': {
           domElement = ownerDocument.createElementNS(MATH_NAMESPACE, type);
           break;
-        case 'script':
-          domElement = createPotentiallyInlineScriptElement(ownerDocument);
+        }
+        case 'script': {
+          // Create the script via .innerHTML so its "parser-inserted" flag is
+          // set to true and it does not execute
+          const div = ownerDocument.createElement('div');
+          if (__DEV__) {
+            if (enableTrustedTypesIntegration && !didWarnScriptTags) {
+              console.error(
+                'Encountered a script tag while rendering React component. ' +
+                  'Scripts inside React components are never executed when rendering ' +
+                  'on the client. Consider using template tag instead ' +
+                  '(https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template).',
+              );
+              didWarnScriptTags = true;
+            }
+          }
+          div.innerHTML = '<script><' + '/script>'; // eslint-disable-line
+          // This is guaranteed to yield a script element.
+          const firstChild = ((div.firstChild: any): HTMLScriptElement);
+          domElement = div.removeChild(firstChild);
           break;
-        case 'select':
-          domElement = createSelectElement(props, ownerDocument);
+        }
+        case 'select': {
+          if (typeof props.is === 'string') {
+            domElement = ownerDocument.createElement('select', {is: props.is});
+          } else {
+            // Separate else branch instead of using `props.is || undefined` above because of a Firefox bug.
+            // See discussion in https://github.com/facebook/react/pull/6896
+            // and discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1276240
+            domElement = ownerDocument.createElement('select');
+          }
+          if (props.multiple) {
+            domElement.multiple = true;
+          } else if (props.size) {
+            // Setting a size greater than 1 causes a select to behave like `multiple=true`, where
+            // it is possible that no option is selected.
+            //
+            // This is only necessary when a select in "single selection mode".
+            domElement.size = props.size;
+          }
           break;
-        default:
-          domElement = createHTMLElement(type, props, ownerDocument);
+        }
+        default: {
+          if (typeof props.is === 'string') {
+            domElement = ownerDocument.createElement(type, {is: props.is});
+          } else {
+            // Separate else branch instead of using `props.is || undefined` above because of a Firefox bug.
+            // See discussion in https://github.com/facebook/react/pull/6896
+            // and discussion in https://bugzilla.mozilla.org/show_bug.cgi?id=1276240
+            domElement = ownerDocument.createElement(type);
+          }
+
+          if (__DEV__) {
+            if (type.indexOf('-') === -1) {
+              // We're not SVG/MathML and we don't have a dash, so we're not a custom element
+              // Even if you use `is`, these should be of known type and lower case.
+              if (type !== type.toLowerCase()) {
+                console.error(
+                  '<%s /> is using incorrect casing. ' +
+                    'Use PascalCase for React components, ' +
+                    'or lowercase for HTML elements.',
+                  type,
+                );
+              }
+              if (
+                // $FlowFixMe[method-unbinding]
+                Object.prototype.toString.call(domElement) ===
+                  '[object HTMLUnknownElement]' &&
+                !hasOwnProperty.call(warnedUnknownTags, type)
+              ) {
+                warnedUnknownTags[type] = true;
+                console.error(
+                  'The tag <%s> is unrecognized in this browser. ' +
+                    'If you meant to render a React component, start its name with ' +
+                    'an uppercase letter.',
+                  type,
+                );
+              }
+            }
+          }
+        }
       }
   }
   precacheFiberNode(internalInstanceHandle, domElement);
@@ -429,7 +525,9 @@ export function createTextInstance(
     const hostContextDev = ((hostContext: any): HostContextDev);
     validateDOMNesting(null, text, hostContextDev.ancestorInfo);
   }
-  const textNode: TextInstance = createTextNode(text, rootContainerInstance);
+  const textNode: TextInstance = getOwnerDocumentFromRootContainer(
+    rootContainerInstance,
+  ).createTextNode(text);
   precacheFiberNode(internalInstanceHandle, textNode);
   return textNode;
 }
diff --git a/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js b/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js
index 92e36b9171515..baa33f1a7c5c6 100644
--- a/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js
+++ b/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js
@@ -35,7 +35,7 @@ import {
   processDispatchQueue,
   accumulateTwoPhaseListeners,
 } from '../DOMPluginEventSystem';
-import isCustomComponent from '../../shared/isCustomComponent';
+import isCustomElement from '../../shared/isCustomElement';
 
 function registerEvents() {
   registerTwoPhaseEvent('onChange', [
@@ -312,7 +312,7 @@ function extractEvents(
   } else if (
     enableCustomElementPropertySupport &&
     targetInst &&
-    isCustomComponent(targetInst.elementType, targetInst.memoizedProps)
+    isCustomElement(targetInst.elementType, targetInst.memoizedProps)
   ) {
     getTargetInstFunc = getTargetInstForChangeEvent;
   }
diff --git a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
index 4ede9b9ddba4d..4168b40cb7afc 100644
--- a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
+++ b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
@@ -2483,11 +2483,7 @@ export function pushStartInstance(
       formatContext.insertionMode !== SVG_MODE &&
       formatContext.insertionMode !== MATHML_MODE
     ) {
-      if (
-        type.indexOf('-') === -1 &&
-        typeof props.is !== 'string' &&
-        type.toLowerCase() !== type
-      ) {
+      if (type.indexOf('-') === -1 && type.toLowerCase() !== type) {
         console.error(
           '<%s /> is using incorrect casing. ' +
             'Use PascalCase for React components, ' +
@@ -2608,7 +2604,7 @@ export function pushStartInstance(
       );
     }
     default: {
-      if (type.indexOf('-') === -1 && typeof props.is !== 'string') {
+      if (type.indexOf('-') === -1) {
         // Generic element
         return pushStartGenericElement(target, props, type);
       } else {
diff --git a/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js b/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
index ca469bc792690..e941899ab4d76 100644
--- a/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
+++ b/packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js
@@ -7,7 +7,7 @@
 
 import {BOOLEAN, getPropertyInfo} from './DOMProperty';
 import {ATTRIBUTE_NAME_CHAR} from './isAttributeNameSafe';
-import isCustomComponent from './isCustomComponent';
+import isCustomElement from './isCustomElement';
 import possibleStandardNames from './possibleStandardNames';
 import hasOwnProperty from 'shared/hasOwnProperty';
 import {enableCustomElementPropertySupport} from 'shared/ReactFeatureFlags';
@@ -308,7 +308,7 @@ function warnUnknownProperties(type, props, eventRegistry) {
 }
 
 export function validateProperties(type, props, eventRegistry) {
-  if (isCustomComponent(type, props)) {
+  if (isCustomElement(type, props) || typeof props.is === 'string') {
     return;
   }
   warnUnknownProperties(type, props, eventRegistry);
diff --git a/packages/react-dom-bindings/src/shared/isCustomComponent.js b/packages/react-dom-bindings/src/shared/isCustomElement.js
similarity index 85%
rename from packages/react-dom-bindings/src/shared/isCustomComponent.js
rename to packages/react-dom-bindings/src/shared/isCustomElement.js
index 9ea9ead2a2f0f..39d5d7fe187e0 100644
--- a/packages/react-dom-bindings/src/shared/isCustomComponent.js
+++ b/packages/react-dom-bindings/src/shared/isCustomElement.js
@@ -7,9 +7,9 @@
  * @flow
  */
 
-function isCustomComponent(tagName: string, props: Object): boolean {
+function isCustomElement(tagName: string, props: Object): boolean {
   if (tagName.indexOf('-') === -1) {
-    return typeof props.is === 'string';
+    return false;
   }
   switch (tagName) {
     // These are reserved SVG and MathML elements.
@@ -30,4 +30,4 @@ function isCustomComponent(tagName: string, props: Object): boolean {
   }
 }
 
-export default isCustomComponent;
+export default isCustomElement;
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js
index 75c5dd361353a..31e4e94ac8c4a 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js
@@ -34,7 +34,7 @@ function initModules() {
   };
 }
 
-const {resetModules, itRenders, clientCleanRender, clientRenderOnServerString} =
+const {resetModules, itRenders, clientCleanRender} =
   ReactDOMServerIntegrationUtils(initModules);
 
 describe('ReactDOMServerIntegration', () => {
@@ -655,22 +655,31 @@ describe('ReactDOMServerIntegration', () => {
       expect(e.getAttribute('class')).toBe('test');
     });
 
+    itRenders('className for is elements', async render => {
+      const e = await render(<div is="custom-element" className="test" />, 0);
+      expect(e.getAttribute('className')).toBe(null);
+      expect(e.getAttribute('class')).toBe('test');
+    });
+
     itRenders('className for custom elements', async render => {
+      const e = await render(<custom-element className="test" />, 0);
       if (ReactFeatureFlags.enableCustomElementPropertySupport) {
-        const e = await render(
-          <div is="custom-element" className="test" />,
-          render === clientRenderOnServerString ? 1 : 0,
-        );
         expect(e.getAttribute('className')).toBe(null);
         expect(e.getAttribute('class')).toBe('test');
       } else {
-        const e = await render(<div is="custom-element" className="test" />, 0);
         expect(e.getAttribute('className')).toBe('test');
+        expect(e.getAttribute('class')).toBe(null);
       }
     });
 
-    itRenders('htmlFor attribute on custom elements', async render => {
+    itRenders('htmlFor property on is elements', async render => {
       const e = await render(<div is="custom-element" htmlFor="test" />);
+      expect(e.getAttribute('htmlFor')).toBe(null);
+      expect(e.getAttribute('for')).toBe('test');
+    });
+
+    itRenders('htmlFor attribute on custom elements', async render => {
+      const e = await render(<custom-element htmlFor="test" />);
       expect(e.getAttribute('htmlFor')).toBe('test');
       expect(e.getAttribute('for')).toBe(null);
     });