An edit from TC53 review that Aki missed (#110)

Author: gesa

index.bs

@@ -59,9 +59,9 @@ Conformance {#conformance}
 
 A conforming implementation of the Minimum Common Web Platform API shall conform to ECMA-262, and additionally shall provide the interfaces and properties listed in this Standard, according to their definition in the corresponding W3C or WHATWG standard.
 
-Runtime-specific extensions to any Web Platform API may be implemented by conforming runtimes. Such extensions shall be defined so that their use neither contradicts, nor causes the non-conformance of, normative functionality of any Web Platform API. It is important to carefully consider use of such extensions, as it reduces interoperability and portability of code across runtimes.
+Runtime-specific extensions to any Web Platform API may be implemented by conforming runtimes. Such extensions shall be defined so that their use neither contradicts, nor causes the non-conformance of, normative functionality of any Web Platform API. Extending API surface, even without modifying existing API behaviours, should be avoided as it can reduce interoperability and portability of code across runtimes.
 
-This Standard does not prohibit implementing additional Web Platform APIs beyond those listed here.
+Runtimes may implement Web Platform APIs beyond those listed in this Standard.
 
 Note: For example, the {{Performance}} API could be extended with additional methods or properties beyond those defined in the [[HR-TIME]] specifications, such as those defined in the [[PERFORMANCE-TIMELINE]] or [[USER-TIMING]] specifications.