1. The menu button doesn't get focused (or they disabled the style that makes focus apparent). It's not possible to open it with the keyboard either by pressing space or by pressing enter.

2. Tab a few more times until you get past the BEM and SMACSS links. Notice how those three tabs can't be selected with the keyboard?

3. Sign up button has no obvious focus.

4. Dropdown menu cannot be accessed with keyboard nor does it have a focus style. The same applies for the buttons in that area as well.

5. Many of the elements in the "Unbelievable Breadth" section either don't allow you to focus or have styled the focus in a really non-obvious way.

Basically, it looks pretty and you can use your clicker, but Semantic "UI" screams to me "works for me so fuck everyone else"