[css-values] Per WG resolution, make zero angles have optional dimension identifier

Author: fantasai

css-values/Overview.bs

@@ -1063,6 +1063,8 @@ Other Quantities</h2>
 Angle Units: the <<angle>> type and ''deg'', ''grad'', ''rad'', ''turn'' units</h3>
 
 	Angle values are <a>dimensions</a> denoted by <dfn id="angle-value">&lt;angle></dfn>.
+	However, for zero angles the unit identifier is optional
+	(i.e. can be syntactically represented as the <<number>> ''0'').
 	The angle unit identifiers are:
 
 	<dl export>
@@ -2076,6 +2078,13 @@ Acknowledgments</h2>
 <h2 class="no-num" id="changes">
 Changes</h2>
 
+	Changes since the <a href="https://www.w3.org/TR/2015/CR-css-values-3-20150611/">11 June 2015 Candidate Recomendation</a> are:
+	
+	<ul>
+		<li>Allow zero angles to be represented as ''0''.
+		    (Change due to Web-compatibility constraints in transform and gradient syntaxes.)
+	</ul>
+
 	Changes since the <a href="https://www.w3.org/TR/2013/CR-css3-values-20130730/">30 July 2013 Candidate Recommendation</a> are:
 
 	<ul>