Add new note about thin/unusual fonts that nominally pass but in prac…

…tice fail

understanding/20/contrast-minimum.html

@@ -62,6 +62,15 @@ <h2>Intent of Contrast (Minimum)</h2>
             ratio, the computed values should not be rounded (e.g. 4.499:1 would not meet the 4.5:1 threshold).</p>
       </div>
 
+      <div class="note">
+         <p>Due to anti-aliasing, particularly thin or unusual fonts may be rendered by user agents with a much fainter
+            color than the actual text color defined in the underlying CSS. This can lead to situations where text has
+            a contrast ratio that nominally passes the Success Criterion, but has a much lower contrast in practice.
+            For these cases, we recommend as a best practice that authors choose a font with stronger/thicker lines,
+            or aim for a text color that exceeds the baseline requirements of this Success Criterion.
+         </p>
+      </div>
+
       <p>The contrast requirements for text also apply to images of text
          (text that has been rendered into pixels and then stored in an image format) - see
          <a href="images-of-text">Success Criterion 1.4.5: Images of Text</a>.