diff --git a/lib/README.md b/lib/README.md
index 40483f2b7..ebca07a42 100644
--- a/lib/README.md
+++ b/lib/README.md
@@ -96,6 +96,10 @@ The following scripts can be executed manually or as part of a GitHub Action:
   - a copy of the common [`styles`](../styles) folder
   - a copy of the document-specific `*/images` folder, if this exists.
 - [`npm run pdf`](build-pdf.mjs) runs the PDF conversion and writes the PDF document into the [`docs/*`](../docs) folder.
+- [`npm run select <CSS selector> [<XPath expression>]`](selector.mjs) selects parts of the generated HTML documents by executing a CSS selector and optionally an XPath expression relative to each match. For example, syntax errors in JSON code snippets can be detected with
+  ```sh
+  npm run select ".json .er" "self::*[.!='…']/text()"
+  ```
 - [`npm test`](../test) runs a test suite.
 
 ## A note on diagrams
diff --git a/lib/selector.mjs b/lib/selector.mjs
new file mode 100644
index 000000000..30233f131
--- /dev/null
+++ b/lib/selector.mjs
@@ -0,0 +1,53 @@
+import puppeteer from "puppeteer";
+import iterator from "./iterator.js";
+
+var docs = [];
+iterator(function (srcname, name, variant) {
+  docs.push(name);
+});
+var browser = await puppeteer.launch({ headless: "new" });
+var exit_code = 0;
+for (var name of docs) {
+  var heading = undefined;
+  console.group(name);
+  var page = await browser.newPage();
+  await page.goto(`${import.meta.dirname}/../docs/${name}/${name}.html`, {
+    waitUntil: "networkidle2",
+  });
+  for (var r of await Promise.all(
+    (
+      await Promise.all(
+        (await page.$$(process.argv[2])).map(async function (elem) {
+          var elems = process.argv[3]
+            ? await elem.$$("xpath/" + process.argv[3])
+            : [elem];
+          if (elems.length > 0) exit_code = 1;
+          return elems.map((elem) =>
+            elem.evaluate(function (e) {
+              return {
+                heading: document.evaluate(
+                  "preceding::*[self::h1|self::h2|self::h3|self::h4|self::h5|self::h6][1]",
+                  e,
+                  () => {},
+                  XPathResult.FIRST_ORDERED_NODE_TYPE,
+                ).singleNodeValue.textContent,
+                match:
+                  e.nodeType === Node.ELEMENT_NODE ? e.outerHTML : e.nodeValue,
+              };
+            }),
+          );
+        }),
+      )
+    ).flat(),
+  )) {
+    if (r.heading !== heading) {
+      if (heading) console.groupEnd();
+      console.group(r.heading);
+      heading = r.heading;
+    }
+    console.log(r.match);
+  }
+  if (heading) console.groupEnd();
+  console.groupEnd();
+}
+process.exit(exit_code);
diff --git a/package.json b/package.json
index 3b3813213..2f71ce2cf 100644
--- a/package.json
+++ b/package.json
@@ -7,6 +7,7 @@
     "build": "node lib/build.js",
     "pdf": "node lib/build-pdf.js",
     "start": "node lib/server",
+    "select": "node lib/selector.mjs",
     "test": "c8 -r html -r text mocha",
     "clean-xxx": "node lib/clean.mjs odata-xxx/temp odata-xxx-v4.0"
   },