Here a quick example how you can create a keyboard button with CSS.
First of all, the text that is to be displayed on the key should be enclosed with a span tag and a class, in HTML code. Something like this to mimic a "Enter" key (can be anything for that matter - even a <i class="icon-xyz"></i> as used by IcoMoon can be encapsulated in that <span>, for example to display the Windows key):
Obviously, you can use a different HTML tag, but for in-text display "span" is one of the usual ones.
The CSS for a of span "keyboardkey" is then defined as something like this: