diff --git a/components/net/fetch/methods.rs b/components/net/fetch/methods.rs
index a073056ee56..b6b5d80231c 100644
--- a/components/net/fetch/methods.rs
+++ b/components/net/fetch/methods.rs
@@ -559,10 +559,10 @@ pub fn should_be_blocked_due_to_nosniff(request_type: Type, response_headers: &H
             mime!(Text / ("x-javascript")),
         ];
 
-        javascript_mime_types.contains(mime_type)
+        javascript_mime_types.iter()
+            .any(|mime| mime.0 == mime_type.0 && mime.1 == mime_type.1)
     }
 
-    let text_css: Mime = mime!(Text / Css);
     // Assumes str::starts_with is equivalent to mime::TopLevel
     return match request_type {
         // Step 6
@@ -575,8 +575,8 @@ pub fn should_be_blocked_due_to_nosniff(request_type: Type, response_headers: &H
         // Step 7
         Type::Style => {
             match content_type_header {
-                Some(&ContentType(ref mime_type)) => mime_type != &text_css,
-                None => true
+                Some(&ContentType(Mime(TopLevel::Text, SubLevel::Css, _))) => false,
+                _ => true
             }
         }
         // Step 8
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index 24d4a7ee7da..e6879b7492e 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -558502,7 +558502,7 @@
    "testharness"
   ],
   "fetch/nosniff/image.html": [
-   "da847d5aa65fc5f52313b4434a84f1938061c7f6",
+   "2b31b5f05d843c55640943e68224cb36a1670a5e",
    "testharness"
   ],
   "fetch/nosniff/importscripts.html": [
@@ -558510,7 +558510,7 @@
    "testharness"
   ],
   "fetch/nosniff/importscripts.js": [
-   "90f5f826fcac39388c603e10b6b76d01a9238427",
+   "e7c02b40c457cc5347acb5a13d0ce14268b8187d",
    "support"
   ],
   "fetch/nosniff/parsing-nosniff.html": [
@@ -558558,11 +558558,11 @@
    "support"
   ],
   "fetch/nosniff/script.html": [
-   "7701124283e0714ef4229fbc6d63a26f8e5e86b1",
+   "762b6033a5b75465417f9921f7d06781ad036cbe",
    "testharness"
   ],
   "fetch/nosniff/stylesheet.html": [
-   "e2dda6ec5981c40dcad68c933336a0358639eec3",
+   "707b1031fc7594d49b6f1487c8ef71864791ea80",
    "testharness"
   ],
   "fetch/nosniff/worker.html": [
diff --git a/tests/wpt/web-platform-tests/fetch/nosniff/image.html b/tests/wpt/web-platform-tests/fetch/nosniff/image.html
index e478789609c..3273f19fb1e 100644
--- a/tests/wpt/web-platform-tests/fetch/nosniff/image.html
+++ b/tests/wpt/web-platform-tests/fetch/nosniff/image.html
@@ -3,17 +3,25 @@
 <div id=log></div>
 <script>
   // Note: images get always sniffed, nosniff doesn't do anything
-  var passes = ["", "?type=", "?type=x", "?type=x/x", "?type=image/gif", "?type=image/png", "?type=image/png;blah"]
+  var passes = [null, "", "x", "x/x", "image/gif", "image/png", "image/png;blah"]
 
-  passes.forEach(function(urlpart) {
+  const get_url = (mime) => {
+    let url = "resources/image.py"
+    if (mime != null) {
+        url += "?type=" + encodeURIComponent(mime)
+    }
+    return url
+  }
+
+  passes.forEach(function(mime) {
     async_test(function(t) {
       var img = document.createElement("img")
       img.onerror = t.unreached_func("Unexpected error event")
       img.onload = t.step_func_done(function(){
         assert_equals(img.width, 96)
       })
-      img.src = "resources/image.py" + urlpart
+      img.src = get_url(mime)
       document.body.appendChild(img)
-    }, "URL query: " + urlpart)
+    }, "URL query: " + mime)
   })
 </script>
diff --git a/tests/wpt/web-platform-tests/fetch/nosniff/importscripts.js b/tests/wpt/web-platform-tests/fetch/nosniff/importscripts.js
index aeb6154870b..d7b121316af 100644
--- a/tests/wpt/web-platform-tests/fetch/nosniff/importscripts.js
+++ b/tests/wpt/web-platform-tests/fetch/nosniff/importscripts.js
@@ -3,15 +3,26 @@ function log(w) { this.postMessage(w) }
 function f() { log("FAIL") }
 function p() { log("PASS") }
 
-["", "?type=", "?type=x", "?type=x/x"].forEach(function(urlpart) {
+const get_url = (mime, outcome) => {
+  let url = "resources/js.py"
+  if (mime != null) {
+      url += "?type=" + encodeURIComponent(mime)
+  }
+  if (outcome) {
+    url += "&outcome=p"
+  }
+  return url
+}
+
+[null, "", "x", "x/x"].forEach(function(mime) {
   try {
-    importScripts("resources/js.py" + urlpart)
+    importScripts(get_url(mime))
   } catch(e) {
-    (e.name == "NetworkError") ? p() : log("FAIL (no NetworkError exception): " + urlpart)
+    (e.name == "NetworkError") ? p() : log("FAIL (no NetworkError exception): " + mime)
   }
 
 })
-importScripts("resources/js.py?type=text/javascript&outcome=p")
-importScripts("resources/js.py?type=text/ecmascript&outcome=p")
-importScripts("resources/js.py?type=text/ecmascript;blah&outcome=p")
+importScripts(get_url("text/javascript", true))
+importScripts(get_url("text/ecmascript", true))
+importScripts(get_url("text/ecmascript;blah", true))
 log("END")
diff --git a/tests/wpt/web-platform-tests/fetch/nosniff/script.html b/tests/wpt/web-platform-tests/fetch/nosniff/script.html
index 667f3c99a6c..aec5a0886f5 100644
--- a/tests/wpt/web-platform-tests/fetch/nosniff/script.html
+++ b/tests/wpt/web-platform-tests/fetch/nosniff/script.html
@@ -4,29 +4,40 @@
 <script>
   var log = function() {}, // see comment below
       p = function() {}, // see comment below
-      fails = ["", "?type=", "?type=x", "?type=x/x"],
-      passes = ["?type=text/javascript", "?type=text/ecmascript", "?type=text/ecmascript;blah"]
+      fails = [null, "", "x", "x/x"],
+      passes = ["text/javascript", "text/ecmascript", "text/ecmascript;blah"]
 
   // Ideally we'd also check whether the scripts in fact execute, but that would involve
   // timers and might get a bit racy without cross-browser support for the execute events.
 
-  fails.forEach(function(urlpart) {
+  const get_url = (mime, outcome) => {
+    let url = "resources/js.py"
+    if (mime != null) {
+        url += "?type=" + encodeURIComponent(mime)
+    }
+    if (outcome) {
+      url += "&outcome=p"
+    }
+    return url
+  }
+
+  fails.forEach(function(mime) {
     async_test(function(t) {
       var script = document.createElement("script")
       script.onerror = t.step_func_done(function(){})
       script.onload = t.unreached_func("Unexpected load event")
-      script.src = "resources/js.py" + urlpart
+      script.src = get_url(mime)
       document.body.appendChild(script)
-    }, "URL query: " + urlpart)
+    }, "URL query: " + mime)
   })
 
-  passes.forEach(function(urlpart) {
+  passes.forEach(function(mime) {
     async_test(function(t) {
       var script = document.createElement("script")
       script.onerror = t.unreached_func("Unexpected error event")
       script.onload = t.step_func_done(function(){})
-      script.src = "resources/js.py" + urlpart + "&outcome=p"
+      script.src = get_url(mime, true)
       document.body.appendChild(script)
-    }, "URL query: " + urlpart)
+    }, "URL query: " + mime)
   })
 </script>
diff --git a/tests/wpt/web-platform-tests/fetch/nosniff/stylesheet.html b/tests/wpt/web-platform-tests/fetch/nosniff/stylesheet.html
index 0ad04038e20..9e47f757b97 100644
--- a/tests/wpt/web-platform-tests/fetch/nosniff/stylesheet.html
+++ b/tests/wpt/web-platform-tests/fetch/nosniff/stylesheet.html
@@ -3,28 +3,36 @@
 <script src=/resources/testharnessreport.js></script>
 <div id=log></div>
 <script>
-  var fails = ["", "?type=", "?type=x", "?type=x/x"],
-      passes = ["?type=text/css", "?type=text/css;blah"]
+  var fails = [null, "", "x", "x/x"],
+      passes = ["text/css", "text/css;charset=utf-8", "text/css;blah"]
 
-  fails.forEach(function(urlpart) {
+  const get_url = (mime) => {
+    let url = "resources/css.py"
+    if (mime != null) {
+        url += "?type=" + encodeURIComponent(mime)
+    }
+    return url
+  }
+
+  fails.forEach(function(mime) {
     async_test(function(t) {
       var link = document.createElement("link")
       link.rel = "stylesheet"
       link.onerror = t.step_func_done(function(){})
       link.onload = t.unreached_func("Unexpected load event")
-      link.href = "resources/css.py" + urlpart
+      link.href = get_url(mime)
       document.body.appendChild(link)
-    }, "URL query: " + urlpart)
+    }, "URL query: " + mime)
   })
 
-  passes.forEach(function(urlpart) {
+  passes.forEach(function(mime) {
     async_test(function(t) {
       var link = document.createElement("link")
       link.rel = "stylesheet"
       link.onerror = t.unreached_func("Unexpected error event")
       link.onload = t.step_func_done(function(){})
-      link.href = "resources/css.py" + urlpart
+      link.href = get_url(mime)
       document.body.appendChild(link)
-    }, "URL query: " + urlpart)
+    }, "URL query: " + mime)
   })
 </script>