[css-2015] Add subheadings.

Author: fantasai

css-2015/Overview.bs

@@ -382,8 +382,8 @@ Requirements for Responsible Implementation of CSS</h2>
 	the CSSWG recommends the following best practices for the implementation of
 	<a>unstable</a> features and <a>proprietary extensions</a> to CSS:
 
-	<ol>
-		<li>
+	<h4 id="unstable">
+	Experimentation and Unstable Features</h4>
 
 			Issue: Rewritten per gregwhitworth's feedback.
 
@@ -419,7 +419,8 @@ Requirements for Responsible Implementation of CSS</h2>
 			Note: Vendors should consult the WG explicitly and not make assumptions on this point,
 			as a pre-CR spec that hasn't changed in awhile is usually more out-of-date than stable.
 
-		<li>
+		<h4 id="de-facto">
+		Market Pressure and De Facto Standards</h4>
 
 			Issue: Added "should not" against encumbered features, per dbaron's request.
 
@@ -470,7 +471,8 @@ Requirements for Responsible Implementation of CSS</h2>
 			that cannot be freely implemented by their competitors
 			(due to platform dependency, licensing restrictions, etc.).</ins>
 
-		<li>
+		<h4 id="proprietary">
+		Proprietary and Non-standardized Features</h4>
 
 			Issue: Rewritten as open vs. closed systems instead of Web vs. non-Web, per szilles' request.
 
@@ -501,7 +503,6 @@ Requirements for Responsible Implementation of CSS</h2>
 			Even if a feature is intended to eventually be used in the Web,
 			if it hasn't yet been standardized
 			it should still not be exposed to the Web.
-	</ol>
 
 
 <h3 id="testing">Implementations of CR-level Features</h3>