diff --git a/components/net/http_loader.rs b/components/net/http_loader.rs
index d1e0bef1068..07e1153a4e1 100644
--- a/components/net/http_loader.rs
+++ b/components/net/http_loader.rs
@@ -523,7 +523,16 @@ pub fn modify_request_headers(headers: &mut Headers,
         port: doc_url.port_or_default()
     };
     headers.set(host);
-    headers.set(UserAgent(user_agent.to_owned()));
+
+    // If the user-agent has not already been set, then use the
+    // browser's default user-agent or the user-agent override
+    // from the command line. If the user-agent is set, don't
+    // modify it, as setting of the user-agent by the user is
+    // allowed.
+    // https://fetch.spec.whatwg.org/#concept-http-network-or-cache-fetch step 8
+    if !headers.has::<UserAgent>() {
+        headers.set(UserAgent(user_agent.to_owned()));
+    }
 
     set_default_accept(headers);
     set_default_accept_encoding(headers);
diff --git a/tests/wpt/metadata/XMLHttpRequest/preserve-ua-header-on-redirect.htm.ini b/tests/wpt/metadata/XMLHttpRequest/preserve-ua-header-on-redirect.htm.ini
deleted file mode 100644
index 468b61c3512..00000000000
--- a/tests/wpt/metadata/XMLHttpRequest/preserve-ua-header-on-redirect.htm.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[preserve-ua-header-on-redirect.htm]
-  type: testharness
-  [XMLHttpRequest: User-Agent header is preserved on redirect 1]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/XMLHttpRequest/setrequestheader-header-allowed.htm.ini b/tests/wpt/metadata/XMLHttpRequest/setrequestheader-header-allowed.htm.ini
deleted file mode 100644
index 04d3654a455..00000000000
--- a/tests/wpt/metadata/XMLHttpRequest/setrequestheader-header-allowed.htm.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[setrequestheader-header-allowed.htm]
-  type: testharness
-  [XMLHttpRequest: setRequestHeader() - headers that are allowed (User-Agent)]
-    expected: FAIL
-