Add anti-aliasing note to 1.4.11 (#3560)

Matches the anti-aliasing notes we recently included for 1.4.3 and 1.4.6
(see #3020)

---------

Co-authored-by: Mike Gower <[email protected]>
Co-authored-by: David Cox <[email protected]>

understanding/21/non-text-contrast.html

@@ -29,6 +29,18 @@ <h2>Intent</h2>
 				<p>The 3:1 contrast ratios referenced in this Success Criterion is intended to be treated as threshold values. When comparing the computed contrast ratio to the Success Criterion ratio, the computed values should not be rounded (e.g. 2.999:1 would not meet the 3:1 threshold).</p>
 			</div>
 
+			<div class="note">
+				<p>Because authors do not have control over user settings for font smoothing and anti-aliasing, when evaluating this
+					 Success Criterion, refer to the colors obtained from the user agent, or the underlying
+					 markup and stylesheets, rather than the non-text elements as presented on screen.</p>
+				<p>Due to anti-aliasing, particularly thin lines and shapes of non-text elements may be rendered by user agents with
+					 a much fainter color than the actual color defined in the underlying CSS. This can lead to situations where
+					 non-text elements have a contrast ratio that nominally passes the Success Criterion, but have a much lower contrast
+					 in practice. In these cases, best practice would be for authors to avoid particularly thin lines and shapes,
+					 or to use a combination of colors that exceeds the normative requirements of this Success Criterion.
+				</p>
+		 </div>
+
 			<section id="user-interface-components">
 				<h3>User Interface Components</h3>