Added tt to code, kbd, pre…

Normalise `<tt>` element to 1em.

Testcase: http://codepen.io/leevigraham/pen/ILAca

The `<tt>` is actually obsolete which make make this invalid. It is however outputted as part of the Sphinx-Doc doc generator which is used by quite a few OS projects. 

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/tt