[css-text-3] Normatively reference Unicode. w3c#3474

Author: fantasai

css-text-3/Overview.bs

@@ -345,6 +345,17 @@ Characters and Letters</h3>
   Authors are forewarned that dividing <a>grapheme clusters</a>
   by element boundaries may give inconsistent or undesired results.
 
+<h3 id="text-encoding">
+Text Processing</h3>
+
+  CSS is built on [[UNICODE]].
+  UAs that support Unicode must adhere to all normative requirements
+  of the Unicode Core Standard,
+  except where explicitly overridden by CSS.
+  UAs that use a different encoding are not explicitly supported by the CSS specifications;
+  they are, however, expected to fulfill the same text handling requirements
+  by assuming an appropriate mapping between that encoding and Unicode.
+
 <h2 id="transforming">
   Transforming Text</h2>