There’s no way to access the shadow root of native HTML 5 elements.
Not useful in this case, but with Chrome it’s possible to access a custom created shadow root:
var root = document.querySelector("#test_button").createShadowRoot();
root.innerHTML = "<button id='inner_button'>Button in button</button"
<button id="test_button"></button>
The root can then be accessed this way:
var element = document.querySelector("#test_button").shadowRoot;
If you want to automate a click on the inner button with selenium python (chromedriver version 2.14+):
>>> outer = driver.execute_script('return document.querySelector("#test_button").shadowRoot')
>>> inner = outer.find_element_by_id("inner_button")
>>> inner.click()
Update 9 Jun 2015
This is the link to the current Shadow DOM W3C Editor’s draft on github:
http://w3c.github.io/webcomponents/spec/shadow/
If you’re interested in browsing the blink source code, this is a good starting point.