diff --git a/fetch/content-length/parsing.window.js b/fetch/content-length/parsing.window.js
new file mode 100644
index 00000000000000..73fbb482e042c1
--- /dev/null
+++ b/fetch/content-length/parsing.window.js
@@ -0,0 +1,19 @@
+promise_test(() => {
+  return fetch("resources/content-lengths.json").then(res => res.json()).then(runTests);
+}, "Loading JSON…");
+
+function runTests(testUnits) {
+  testUnits.forEach(testUnit => {
+    const input = encodeURIComponent(testUnit.input);
+    promise_test(t => {
+      const result = fetch("resources/content-length.py?length=" + input);
+      if (testUnit.output === null) {
+        return promise_rejects_js(t, TypeError, result);
+      } else {
+        return result.then(res => res.text()).then(text => {
+          assert_equals(text.length, testUnit.output);
+        });
+      }
+    }, input);
+  });
+}
diff --git a/fetch/content-length/resources/content-length.py b/fetch/content-length/resources/content-length.py
new file mode 100644
index 00000000000000..b4583b8da37197
--- /dev/null
+++ b/fetch/content-length/resources/content-length.py
@@ -0,0 +1,9 @@
+def main(request, response):
+    response.add_required_headers = False
+    output =  b"HTTP/1.1 200 OK\r\n"
+    output += b"Content-Type: text/plain;charset=UTF-8\r\n"
+    output += request.GET.first(b"length") + b"\r\n"
+    output += b"\r\n"
+    output += b"Fact: this is really forty-two bytes long."
+    response.writer.write(output)
+    response.close_connection = True
diff --git a/fetch/content-length/resources/content-lengths.json b/fetch/content-length/resources/content-lengths.json
new file mode 100644
index 00000000000000..dac9c82dc09784
--- /dev/null
+++ b/fetch/content-length/resources/content-lengths.json
@@ -0,0 +1,126 @@
+[
+  {
+    "input": "Content-Length: 42",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 42,42",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 42\r\nContent-Length: 42",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 42\r\nContent-Length: 42,42",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 30",
+    "output": 30
+  },
+  {
+    "input": "Content-Length: 30,30",
+    "output": 30
+  },
+  {
+    "input": "Content-Length: 30\r\nContent-Length: 30",
+    "output": 30
+  },
+  {
+    "input": "Content-Length: 30\r\nContent-Length: 30,30",
+    "output": 30
+  },
+  {
+    "input": "Content-Length: 42,30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: 30,42",
+    "output": null
+  },
+  {
+    "input": "Content-Length: 42\r\nContent-Length: 30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: 30\r\nContent-Length: 42",
+    "output": null
+  },
+  {
+    "input": "Content-Length: 30,",
+    "output": null
+  },
+  {
+    "input": "Content-Length: ,30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: 30\r\nContent-Length: \t",
+    "output": null
+  },
+  {
+    "input": "Content-Length: \r\nContent-Length: 30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: aaaah\r\nContent-Length: nah",
+    "output": null
+  },
+  {
+    "input": "Content-Length: aaaah, nah",
+    "output": null
+  },
+  {
+    "input": "Content-Length: aaaah\r\nContent-Length: aaaah",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: aaaah, aaaah",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: aaaah",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 42s",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 30s",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: -1",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 0x20",
+    "output": 42
+  },
+  {
+    "input": "Content-Length: 030",
+    "output": 30
+  },
+  {
+    "input": "Content-Length: 030\r\nContent-Length: 30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: 030, 30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: \"30\"",
+    "output": 42
+  },
+  {
+    "input": "Content-Length:30\r\nContent-Length:,\r\nContent-Length:30",
+    "output": null
+  },
+  {
+    "input": "Content-Length: ",
+    "output": 42
+  }
+]
diff --git a/fetch/content-length/too-long.window.js b/fetch/content-length/too-long.window.js
new file mode 100644
index 00000000000000..f8cefaa9c23fbf
--- /dev/null
+++ b/fetch/content-length/too-long.window.js
@@ -0,0 +1,4 @@
+promise_test(async t => {
+  const result = await fetch(`resources/content-length.py?length=${encodeURIComponent("Content-Length: 50")}`);
+  await promise_rejects_js(t, TypeError, result.text());
+}, "Content-Length header value of network response exceeds response body");