Re-introduce the 'at least two independent implementations' SHOULD fr…

…om the previous version of the charter.

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@@ -296,6 +296,13 @@ <h3>Requirements for advancing the maturity level of features</h3>
     <p>Within the requirements of the Process each new normative feature
     will be considered as ready to advance to a higher maturity level if
     its implementability has been demonstrated.</p>
+    <p>In order to advance to <a
+    href="https://www.w3.org/Consortium/Process/#RecsPR"
+    title="Proposed Recommendation">Proposed Recommendation</a>, each
+    Recommendation-track Technical Report SHOULD have <a
+    href="https://www.w3.org/Consortium/Process/#implementation-experience">at
+    least two independent implementations</a> of each feature defined in
+    the Technical Report.</p>
     <p>When considering suitability to advance any feature beyond
     Candidate Recommendation, at least two independent factors of verification
     MUST be demonstrated, which may come from any of:</p>