Contrast minimum: Address the active (pressed) state

From #157: the active state (the typically split-second state when a button or control is being clicked or pressed) is not explicitly required to meet the contrast requirement.

understanding/20/contrast-minimum.html

@@ -119,7 +119,9 @@ <h2>Intent of Contrast (Minimum)</h2>
       <p>This Success Criterion applies to text in the page, including
          placeholder text and text that is shown when a pointer is hovering over an object
          or when an object has keyboard focus. If any of these are used in a page, the text
-         needs to provide sufficient contrast.
+         needs to provide sufficient contrast. The active state (the typically split-second
+         state when a button or control is being clicked or pressed) is not explicitly
+         required to meet the contrast requirement.
       </p>
 
       <p>Although this Success Criterion only applies to text, similar issues occur for content presented