Add Form Control Focus Indication

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Currently form-control components have their outline suppressed but do not provide an alternative style using the :focus pseudo-selector, the result of which is a usability issue.

Without outline or :focus, it's not possible to tell where the cursor is while navigating form elements using switch-based peripherals such as tab, or when shifting contextual frames of reference such as navigating through browser history.

Here's what a focused button looks like using a standard outline:

And here's what that same button looks like with focus in [email protected]:

This usability issue is exacerbated when using button groups and/or groups of form controls. It is not currently a problem for a elements, which maintain their focus when navigating using a

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Proposal is to add :focus with a slight change in alpha transparency, the value of which may be driven by the amount of WCAG 2.0 compliance desired by the library.