So, let's inspect what has just happened. You saw that hello world, not very impressive but if you open your DevTools or Firebug, in the browser you would see that that h1 has data-reactroot, so I'm using version 15, if you're an older version like version 0.13, then you would see a different attribute but I assume you use version 15 which I used in my include to the CDN, so this will tell you that this element hasn't been there initially, it has been added by React. So, if you go to Instagram, if you go to Facebook, you can find reactroot. So, this is an attribute which you can also use in your stylesheet when you want to style those elements, created basically and mounted. So, every time you mount something with ReactDOM, that root element will have this attribute. So, just remember, write it down somewhere, data-reactroot is automatically given to you.