[css-color] Warn against defining your stringifiers for your own classes.

Author: tabatkins

css-color/Overview.bs

@@ -3154,9 +3154,13 @@ Adding New Color Classes</h4>
 			and a <code>toTheOtherColor()</code> method on <code>MyNewColor.prototype</code>.
 
 		<li>
-			Custom stringifiers on the <code>stringifiers</a> object,
-			if it's more accurate/efficient to convert your color class into one of the stringifier types directly
+			Custom stringifiers on the <code>stringifiers</a> object for one of the UA-defined color classes,
+			if it's more accurate/efficient to convert your color class into one of the UA-defined types directly
 			than it is to first convert it into an <a idl>RGBColor</a>.
+
+			Note: It is <strong>not</strong> generally useful to define a special stringifier for your class itself;
+			the main purpose of the stringifier is to allow the object to be used in a CSS property,
+			and only the UA-defined color classes correspond to valid CSS syntaxes.
 	</ul>
 
 	All of these apply to our example HSV color class,

css-color/Overview.html

@@ -3262,9 +3262,13 @@ <h4 class="heading settled heading" data-level=15.5.2 id=api-adding-classes><spa
 			and a <code>toTheOtherColor()</code> method on <code>MyNewColor.prototype</code>.
 
 		<li>
-			Custom stringifiers on the <code>stringifiers object,
-			if it’s more accurate/efficient to convert your color class into one of the stringifier types directly
+			Custom stringifiers on the <code>stringifiers object for one of the UA-defined color classes,
+			if it’s more accurate/efficient to convert your color class into one of the UA-defined types directly
 			than it is to first convert it into an <a data-link-type=idl href=#dom-rgbcolor title=rgbcolor>RGBColor</a>.
+
+<p class=note>Note: It is <strong>not</strong> generally useful to define a special stringifier for your class itself;
+			the main purpose of the stringifier is to allow the object to be used in a CSS property,
+			and only the UA-defined color classes correspond to valid CSS syntaxes.</p>
 	</code></ul><code>
 
 <p>All of these apply to our example HSV color class,