**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
React 15.x was able to render within a DocumentFragment/shadowRoot, not possible with Fiber anymore

**DEMO**

https://www.webpackbin.com/bins/-KvPFG7-HGfQ34IgUxQt

```js
import React, {Component} from 'react'
import {render} from 'react-dom'


class MyComponent extends Component{
  render(){
    return <div>Hello From React !</div>
  }
}

class MyElement extends HTMLElement {
  static is = 'my-element'
  constructor(){
    super()
    const shadowRoot = this.attachShadow({mode:'open'})
    try {
      // this fails 
      render(<MyComponent/>, shadowRoot)
    } catch (e){
      console.error(e)
      shadowRoot.innerHTML = `
        <div style="color:red;">${e}</div>
        <div>see console log for stack trace</div>
       `
    }
  }
  connectedCallback(){
    console.log('MyElement mounted!')
  }
}

customElements.define(MyElement.is, MyElement)
```


related -> https://github.com/skatejs/renderer-react/issues/3

**What is the expected behavior?**
be able to render to shadow root like previous versions of React

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16
Chrome

switch (c) {
       case "&":
        return "&amp;";
       case "<":
        return "&lt;";
       case ">":
        return "&gt;";
       case '"':
        return "&quot;";
       case " ":
        return "&nbsp;";
      }
    }
    function escapeAttr(s) {
      return s.replace(escapeAttrRegExp, escapeReplace);
    }
    function escapeData(s) {
      return s.replace(escapeDataRegExp, escapeReplace);
    }
    function makeSet(arr) {
      var set = {};
      for (var i = 0; i < arr.length; i++) {
        set[arr[i]] = true;
      }
      return set;
    }
    var voidElements = makeSet([ "area", "base", "br", "col", "command", "embed", "hr", "img", "input", "keygen", "link", "meta", "param", "source", "track", "wbr" ]);
    var plaintextParents = makeSet([ "style", "script", "xmp", "iframe", "noembed", "noframes", "plaintext", "noscript" ]);
    function getOuterHTML(node, parentNode) {