[css-2015] Remove <ins>, add introduction to open standards clause.

Author: fantasai

css-2015/Overview.bs

@@ -493,8 +493,8 @@ Requirements for Responsible Implementation of CSS</h2>
 			<p>When exposing such an <a>unstable</a> feature to the Web in a production release,
 			implementations should support both prefixed and unprefixed syntaxes
 			for the feature.
-			<ins>Once the feature has stabilized and the implementation is updated to match interoperable behavior,
-			support for the prefixed syntax should be removed.</ins>
+			Once the feature has stabilized and the implementation is updated to match interoperable behavior,
+			support for the prefixed syntax should be removed.
 
 			<details class=why>
 				<summary>Why?</summary>
@@ -504,12 +504,13 @@ Requirements for Responsible Implementation of CSS</h2>
 				as they get ironed out through the standards/bugfixing process.
 			</details>
 
-			<p>Vendors should <ins>make it possible for other implementors
+			In order to preserve the open nature of CSS as a technology,
+			vendors should make it possible for other implementors
 			to freely implement any features that they do ship.
-			To this end, they should</ins> provide spec-editing and testing resources
+			To this end, they should provide spec-editing and testing resources
 			to complete standardization of such features,
-			<ins>and avoid other obstacles (e.g., platform dependency, licensing restrictions)
-			to their competitors shipping the feature.</ins>
+			and avoid other obstacles (e.g., platform dependency, licensing restrictions)
+			to their competitors shipping the feature.
 
 
 <h3 id="testing">Implementations of CR-level Features</h3>