diff --git a/packages/endo/src/url.js b/packages/endo/src/url.js
new file mode 100644
index 0000000000..ab591a3c52
--- /dev/null
+++ b/packages/endo/src/url.js
@@ -0,0 +1,78 @@
+// Derrived from https://github.com/junosuarez/url-relative
+//
+// Copyright (c) MMXV jden jason@denizac.org
+//
+// ISC License
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+export const relative = (referrer, location) => {
+  referrer = String(referrer || "");
+  location = String(location || "");
+
+  if (referrer === location) {
+    return "";
+  }
+
+  const referrerURL = new URL(referrer);
+  const locationURL = new URL(location);
+
+  if (referrerURL.host !== locationURL.host) {
+    return location;
+  }
+
+  if (referrerURL.protocol !== locationURL.protocol) {
+    return location;
+  }
+
+  if (referrerURL.auth !== locationURL.auth) {
+    return location;
+  }
+
+  // left location right, look for closest common path segment
+  const referrerParts = referrerURL.pathname.substr(1).split("/");
+  const locationParts = locationURL.pathname.substr(1).split("/");
+
+  if (referrerURL.pathname === locationURL.pathname) {
+    if (locationURL.pathname[locationURL.pathname.length - 1] === "/") {
+      return ".";
+    }
+    return locationParts[locationParts.length - 1];
+  }
+
+  while (referrerParts[0] === locationParts[0]) {
+    referrerParts.shift();
+    locationParts.shift();
+  }
+
+  let length = referrerParts.length - locationParts.length;
+  if (length > 0) {
+    if (referrer.endsWith("/")) {
+      locationParts.unshift("..");
+    }
+    while (length > 0) {
+      length -= 1;
+      locationParts.unshift("..");
+    }
+    return locationParts.join("/");
+  }
+  if (length < 0) {
+    return locationParts.join("/");
+  }
+  length = locationParts.length - 1;
+  while (length > 0) {
+    length -= 1;
+    locationParts.unshift("..");
+  }
+  return locationParts.join("/");
+};
diff --git a/packages/endo/test/relative.test.js b/packages/endo/test/relative.test.js
new file mode 100644
index 0000000000..4637a54803
--- /dev/null
+++ b/packages/endo/test/relative.test.js
@@ -0,0 +1,117 @@
+// Derrived from https://github.com/junosuarez/url-relative
+//
+// Copyright (c) MMXV jden jason@denizac.org
+//
+// ISC License
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+import tape from "tape";
+import { relative } from "../src/url.js";
+
+const { test } = tape;
+
+test("different protocol", t => {
+  t.plan(2);
+  t.equal(
+    relative("http://a.com:12/a", "https://a.com:12/a"),
+    "https://a.com:12/a"
+  );
+  t.equal(
+    relative("http://a.com:12/a/", "https://a.com:12/a/"),
+    "https://a.com:12/a/"
+  );
+});
+
+test("file protocol", t => {
+  t.plan(1);
+  t.equal(relative("file:///a", "file:///b"), "b");
+});
+
+test("different domain", t => {
+  t.plan(2);
+  t.equal(relative("http://a.com:12/a", "http://b.com/a"), "http://b.com/a");
+  t.equal(relative("http://a.com:12/a/", "http://b.com/a/"), "http://b.com/a/");
+});
+
+test("same domain", t => {
+  t.plan(2);
+  t.equal(relative("http://a.com/a", "http://a.com/b"), "b");
+  t.equal(relative("http://a.com/a/", "http://a.com/b/"), "../b/");
+});
+
+test("divergent paths, longer from", t => {
+  t.plan(4);
+  t.equal(
+    relative("http://example.com/a/b/c/d", "http://example.com/a/b/d"),
+    "../d"
+  );
+  t.equal(
+    relative("http://example.com/a/b/c/d/e", "http://example.com/a/d/e"),
+    "../../d/e"
+  );
+  t.equal(
+    relative("http://example.com/a/b/c/d/", "http://example.com/a/b/d/"),
+    "../../d/"
+  );
+  t.equal(
+    relative("http://example.com/a/b/c/d/e/", "http://example.com/a/d/e/"),
+    "../../../d/e/"
+  );
+});
+
+test("divergent paths, longer to", t => {
+  t.plan(6);
+  t.equal(
+    relative("http://example.com/a/b/c/d", "http://example.com/a/b/c/d/e"),
+    "e"
+  );
+  t.equal(
+    relative("http://example.com/a/b/c/d", "http://example.com/a/b/c/d/e/f"),
+    "e/f"
+  );
+  t.equal(relative("http://example.com/", "http://example.com/a/b"), "a/b");
+  t.equal(
+    relative("http://example.com/a/b/c/d/", "http://example.com/a/b/c/d/e/"),
+    "e/"
+  );
+  t.equal(
+    relative("http://example.com/a/b/c/d/", "http://example.com/a/b/c/d/e/f/"),
+    "e/f/"
+  );
+  t.equal(relative("http://example.com/", "http://example.com/a/b/"), "a/b/");
+});
+
+test("divergent paths, equal length", t => {
+  t.plan(2);
+  t.equal(
+    relative(
+      "http://example.com/a/b/c/d/e/f",
+      "http://example.com/a/b/c/g/h/j"
+    ),
+    "../../g/h/j"
+  );
+  t.equal(
+    relative(
+      "http://example.com/a/b/c/d/e/f/",
+      "http://example.com/a/b/c/g/h/j/"
+    ),
+    "../../../g/h/j/"
+  );
+});
+
+test("identical", t => {
+  t.plan(2);
+  t.equal(relative("https://a.com/a", "https://a.com/a"), "");
+  t.equal(relative("https://a.com/a/", "https://a.com/a/"), "");
+});