[css-fonts-4] OT tags are 4 char, fix w3c#5195

Author: svgeesus

css-fonts-4/Overview.bs

@@ -5184,8 +5184,12 @@ Values have the following meanings:
   <dd>specifies that when rendering with OpenType fonts,
       the content language of the element is used to infer the OpenType language system</dd>
   <dt><dfn data-dfn-type=value data-dfn-for=font-language-override id="font-language-override-string-value"><<string>></dfn></dt>
-  <dd>single three-letter case-sensitive OpenType <a href="https://www.microsoft.com/typography/otspec/languagetags.htm">language system tag</a>,
-      specifies the OpenType language system to be used instead of the language system implied by the language of the element</dd>
+  <dd>single four-character case-sensitive OpenType <a href="https://www.microsoft.com/typography/otspec/languagetags.htm">language system tag</a>,
+	  specifies the OpenType language system to be used instead of the language system implied by the language of the element.
+	  If the string is shorter than four characters,
+	  it is padded at the end with space (U+0020) characters
+	  such that the length is 4, before being matched.
+	</dd>
 </dl>
 
 <!-- For this level, string represents a single language code, there's no concept of "fallback"