diff --git a/specs/latest/1.0/index.html b/specs/latest/1.0/index.html
index 1f00be6b3a..34284e8de3 100644
--- a/specs/latest/1.0/index.html
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@@ -28,7 +28,7 @@
     <!--end-logo-->
 
     <h1>WebGL Specification</h1>
-    <h2 class="no-toc">Editor's Draft 14 September 2015</h2>
+    <h2 class="no-toc">Editor's Draft 29 September 2015</h2>
     <dl>
         <dt>This version:
             <dd>
@@ -3840,6 +3840,28 @@ <h3>GLSL ES Global Variable Initialization</h3>
     content.
 </div>
 
+<h3><a name="PREPROCESSOR_DEFINED_IN_MACRO">GLSL ES Preprocessor "defined" Operator Generated By Macro Replacement</a></h3>
+
+<p>
+    The C++ standard, which the GLSL ES preprocessor specification refers to, has undefined behavior
+    when the <code>defined</code> operator is generated by macro replacement when parsing the
+    controlling expression of an <code>#if</code> or <code>#elif</code> directive. When shader code
+    processed by the WebGL API generates the token <code>defined</code> during macro replacement
+    inside a preprocessor expression, that token must be immediately handled as the
+    <code>defined</code> operator before any further macro replacement.
+</p>
+
+<p>
+    This has no effect on macro expansion outside preprocessor directives that handle the
+    <code>defined</code> operator.
+</p>
+
+<div class="note rationale">
+    This behavior is enforced across GLES implementations by dEQP tests, so shaders created on other
+    platforms may rely on it. In addition, the ported dEQP tests in the WebGL conformance suite are
+    only valid if this behavior is specified.
+</div>
+
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     <h2>References</h2>