--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -517677,69 +517677,69 @@
    "cb4bf3851c4064cb650366da300998faf07e37fc",
    "reftest"
   ],
   "css/css-shapes/spec-examples/shape-outside-008.html": [
    "5a9f340648d85f2fdd3cd3fe74b2e145fbc1a54a",
    "reftest"
   ],
   "css/css-shapes/spec-examples/shape-outside-010.html": [
-   "7baf8e86ee451f08ab18e03000d64a529a2824d0",
+   "929078a33a23f1d10ce9d0f89016725f233e133e",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-011.html": [
-   "3620cce0bf178d1bc67faa372406c50b3b07c086",
+   "af178c8f965ce5f48673a4b67aa15f4974a09a2a",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-012.html": [
-   "1db1130bb5f2f5a8a12bab7ec5a9fa0c40165be4",
+   "67e23bdb9873267a12790cf4cd60b704f423f8a5",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-013.html": [
-   "155d33ea047169acdc6b29b4716fa7237b133d51",
+   "7bd3ac39a58324354fa2c36fa98b151ce1292c85",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-014.html": [
-   "ab449eda8aa379e0149c759e9dcd9a866c1ed3db",
+   "e87ca4ce9f46103091139fda326452e4c66b1980",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-015.html": [
-   "f0ce9eb24d3b6bf8b8aa03f5cd02f82acfcdf8cd",
+   "d30174b5995f0831f2535a7a817f6ad5af5a6ba8",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-016.html": [
-   "2ea1538bff7b1825cdd63ea6510435c7aa52aebc",
+   "585b41b91bd9d452b616f87e81c8a47576299719",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-017.html": [
-   "366e0ad5c7ae7f2663f9c6bc6c0eba51e299f120",
+   "9593d57329acf1998d4b5ada5a230f3fed0d3c32",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-018.html": [
-   "08c9fd21d74ae52b98581c7d013275c30d49e18c",
+   "c81ef4303d00a4f490f8cada68d8a6449daa56c3",
    "testharness"
   ],
   "css/css-shapes/spec-examples/shape-outside-019.html": [
-   "46de7762b9a9c29a0aa798775cea5dfb17bbf5a0",
+   "3eaf5c74cfbbb676b3f2ccae55a96d3999ac890b",
    "testharness"
   ],
   "css/css-shapes/spec-examples/support/circle-no-shadow.png": [
    "456a5b15b1866f5a169531639b0cd71c06b31b25",
    "support"
   ],
   "css/css-shapes/spec-examples/support/circle-shadow.png": [
    "6a149114684e9f3b40fb8c2c9452e7862b573c7e",
    "support"
   ],
   "css/css-shapes/spec-examples/support/rounded-triangle.svg": [
    "ef31c9d31d9aa7d27b94cacb7c3b5522c357ea8c",
    "support"
   ],
   "css/css-shapes/spec-examples/support/spec-example-utils.js": [
-   "137b347da1085eb0042f1d2fc017694ff528180b",
+   "de748e548169b360cf0d31a5499d766dd8bc2a7d",
    "support"
   ],
   "css/css-shapes/support/1x1-green.png": [
    "51e7b6974a09eda6cb31337717c5eaeb9c44b443",
    "support"
   ],
   "css/css-shapes/support/1x1-lime.png": [
    "b040eb633a35c0648ad72a2902361faf25bc419d",
@@ -560761,17 +560761,17 @@
    "90d51a5e46cc58404dd5ec1e9e4e10934a6c0707",
    "support"
   ],
   "fetch/api/resources/data.json": [
    "6418064de426ecf8f0e508c751e60a0079ba044d",
    "support"
   ],
   "fetch/api/resources/echo-content.py": [
-   "edc7a1d75d7c438ca3cca59f81d03058b3c2758f",
+   "df8169fcbb5312760a7bff3bd3c9b2e0612a5013",
    "support"
   ],
   "fetch/api/resources/empty.txt": [
    "da39a3ee5e6b4b0d3255bfef95601890afd80709",
    "support"
   ],
   "fetch/api/resources/infinite-slow-response.py": [
    "dee0fa986bf9ce4fd5cd5127a9945caf1b1a4e6f",
@@ -614053,17 +614053,17 @@
    "6614b5b0febd718a94bbec110568b9aaf80dc9eb",
    "testharness"
   ],
   "webrtc/RTCPeerConnection-setRemoteDescription-tracks.https.html": [
    "16fe3b155e55d1b66181788c93e570b36e5cc67d",
    "testharness"
   ],
   "webrtc/RTCPeerConnection-setRemoteDescription.html": [
-   "59f826d4d23cd52f15f3940cc3abe66933e91037",
+   "8a3e2f1e157e1ceed18ac66e57040a941b658f24",
    "testharness"
   ],
   "webrtc/RTCPeerConnection-track-stats.https.html": [
    "c31d22b6d9322d6d9b12414682d04137ad8cfb5d",
    "testharness"
   ],
   "webrtc/RTCPeerConnectionIceEvent-constructor.html": [
    "f273bd7fdfc883a15e8fb16fef5309061254c6cc",

--- a/testing/web-platform/tests/fetch/api/resources/echo-content.py
+++ b/testing/web-platform/tests/fetch/api/resources/echo-content.py
@@ -1,9 +1,10 @@
 def main(request, response):
 
     headers = [("X-Request-Method", request.method),
                ("X-Request-Content-Length", request.headers.get("Content-Length", "NO")),
-               ("X-Request-Content-Type", request.headers.get("Content-Type", "NO"))]
-
+               ("X-Request-Content-Type", request.headers.get("Content-Type", "NO")),
+               # Avoid any kind of content sniffing on the response.
+               ("Content-Type", "text/plain")]
     content = request.body
 
     return headers, content