Some comments.

build-system/amp4test.js

@@ -47,7 +47,7 @@ app.use('/compose-doc', function(req, res) {
   let experimentsBlock = '';
   if (experiments) {
     const string = `"${experiments.split(',').join('","')}"`;
-    // TODO: Why is localDev necessary?
+    // TODO: Why is setting localDev necessary?
     // `allow-doc-opt-in` enables any experiment to be enabled via doc opt-in.
     experimentsBlock =
       `<script>
@@ -57,7 +57,7 @@ app.use('/compose-doc', function(req, res) {
       <meta name="amp-experiments-opt-in" content="${experiments}">`;
   }
 
-  // TODO: What is amp-3p-iframe-src used for?
+  // TODO: Do we need to inject amp-3p-iframe-src for non-ad tests?
   const head =
     `${experimentsBlock}
     <meta name="amp-3p-iframe-src" content="http://localhost:9876/${frameHtml}">`;
@@ -217,7 +217,8 @@ function composeDocument(config) {
     extensionScripts = extensions.map(extension => {
       const src = (cdn)
         ? `https://cdn.ampproject.org/v0/${extension}-0.1.js`
-        : `/dist/v0/${extension}-0.1.${compiled ? '' : 'max.'}js`; // TODO: Use '-latest'?
+        // TODO: Version 0.1 is hard-coded. Use '-latest'?
+        : `/dist/v0/${extension}-0.1.${compiled ? '' : 'max.'}js`;
       return `<script async custom-element="${extension}" src="${src}"></script>`;
     }).join('\n');
   }
@@ -269,6 +270,8 @@ function composeDocument(config) {
   // To enable A4A FIE, a <script amp-ad-metadata> tag must exist.
   let ampAdMeta = '';
   if (amp === 'amp4ads') {
+    // `ampRuntimeUtf16CharOffsets` is used to cut out all runtime scripts,
+    // which are not needed in FIE mode.
     const start = topHalfOfHtml.indexOf(runtimeScript);
     let end = start + runtimeScript.length;
 
@@ -280,7 +283,8 @@ function composeDocument(config) {
       extensionsMap = customElements.map(ce => {
         return {
           'custom-element': ce,
-          'src': `https://cdn.ampproject.org/v0/${ce}-0.1.js`, // TODO: Local?
+          // TODO: Should this be a local URL i.e. /dist/v0/...?
+          'src': `https://cdn.ampproject.org/v0/${ce}-0.1.js`,
         };
       });
     }