This is the same as any other element in CSS, there is a psuedo selector for hovering and focusing, but not specifically for clicks. You can also use the attribute selector:
input[type="reset"]{} input[type="reset"]:hover{} input[type="reset"]:focus{} input[type="reset"]:active{}
The focus style will be applied when the user first presses (or focuses, of course), but loses the style as soon as the user moves the focus to another element. To customize the click style for this small moment when the mouse button is unavailable, use javascript or try the :active selector, which (I just realized now) can be applied to unattached elements.
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