CSS: Justify use of rtrim on CSS property values

CSS does not acknowledge carriage return or form feed characters
as whitespace but it does replace them with whitespace, making it
acceptable to use `rtrim`.

Closes gh-4956

Author: gibson042

src/css/curCSS.js

@@ -16,6 +16,12 @@ function curCSS( elem, name, computed ) {
 
 		// trim whitespace for custom property (issue gh-4926)
 		if ( isCustomProp ) {
+
+			// rtrim treats U+000D CARRIAGE RETURN and U+000C FORM FEED
+			// as whitespace while CSS does not, but this is not a problem
+			// because CSS preprocessing replaces them with U+000A LINE FEED
+			// (which *is* CSS whitespace)
+			// https://www.w3.org/TR/css-syntax-3/#input-preprocessing
 			ret = ret.replace( rtrim, "$1" );
 		}