In Playing with Shadow DOM Chris takes a look at why Twitter moved away from using iframes with their embeddable tweets
TL;DR:

• Shadow DOM is pretty cool.

• It’s comparable to an iframe in many ways, including style encapsulation. Embedded third-party content is a pretty good use case.

• It’s possible to use it while falling back to an iframe pretty easily.

• It’s a part of the larger world of web components, but you don’t have to go all-in on all that if you don’t want to.

Source: Playing with Shadow DOM | CSS-Tricks