[css-text-4] Add auto value to text-spacing

Closes #3229

Author: litherum

css-text-4/Overview.bs

@@ -1873,7 +1873,7 @@ Character Class Spacing: the 'text-spacing' property</h3>
 
 	<pre class="propdef">
 	Name: text-spacing
-	Value: normal | none |
+	Value: normal | none | auto |
 	       [ trim-start | space-start | space-first ] ||
 	       [ trim-end | space-end | allow-end ] ||
 	       [ trim-adjacent | space-adjacent ] ||
@@ -1903,6 +1903,15 @@ Character Class Spacing: the 'text-spacing' property</h3>
 			Turns off all text-spacing features.
 			All fullwidth characters are set with full-width glyphs.
 
+		<dt><dfn>auto</dfn>
+		<dd>
+			The user agent chooses a set of typographically high quality spacing values.
+                        Different user agents running on different platforms may pick different values.
+
+                        Note: These spacing values may or may not match OS platform conventions.
+
+                        Note: The behavior of ''auto'' might not be achievable with any combination of other values of 'text-spacing'.
+
 		<dt><dfn>ideograph-alpha</dfn>
 		<dd>
 			Creates 1/4em extra spacing between runs of