This usually means that the user mixed up our lowercase/uppercase naming convention; see https://github.com/facebook/react/pull/3429#issuecomment-85166440.

We could add a message like

> Warning: The tag <commentBox> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.

We'd check this only once per tag and only in development mode.

var firstChild = ((div.firstChild: any): HTMLScriptElement);
        domElement = div.removeChild(firstChild);
      } else if (props.is) {
        domElement = ownerDocument.createElement(type, props.is);
      } else {
        
        
        
        domElement = ownerDocument.createElement(type);
      }
    } else {
      domElement = ownerDocument.createElementNS(namespaceURI, type);
    }
    return domElement;
  },
  setInitialProperties(
    domElement: Element,
    tag: string,
    rawProps: Object,
    rootContainerElement: Element,
  ): void {
    var isCustomComponentTag = isCustomComponent(tag, rawProps);
    if (__DEV__) {
      validatePropertiesInDevelopment(tag, rawProps);
      if (isCustomComponentTag && !didWarnShadyDOM && domElement.shadyRoot) {
        warning(
          false,
          '%s is using shady DOM. Using shady DOM with React can ' +
            'cause things to break subtly.',
          getCurrentFiberOwnerName() || 'A component',