It's <cxx-postconditions>, not <cxx-postcondition>.

Filed cplusplus/html-doc-framework#12.

Author: jyasskin

optional.html

@@ -533,7 +533,7 @@ <h1>Class <code>bad_optional_access</code></h1>
       <cxx-signature>bad_optional_access();</cxx-signature>
 
       <cxx-effects>Constructs an object of class <code>bad_optional_access</code>.</cxx-effects>
-      <cxx-postcondition><code>what()</code> returns an implementation-defined NTBS.</cxx-postcondition>
+      <cxx-postconditions><code>what()</code> returns an implementation-defined NTBS.</cxx-postconditions>
     </cxx-function>
   </cxx-section>