diff --git a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
index 314f12e2ff..5f5638f7dc 100644
--- a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
+++ b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
@@ -406,8 +406,7 @@ static String normalize_method(String const& method)
 // https://fetch.spec.whatwg.org/#concept-header-value-normalize
 static String normalize_header_value(String const& header_value)
 {
-    // FIXME: I'm not sure if this is the right trim, it should only be HTML whitespace bytes.
-    return header_value.trim_whitespace();
+    return header_value.trim(StringView { http_whitespace_bytes });
 }
 
 // https://xhr.spec.whatwg.org/#dom-xmlhttprequest-setrequestheader
diff --git a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h
index c8a02a01fa..b556aed0b3 100644
--- a/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h
+++ b/Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h
@@ -19,6 +19,8 @@
 
 namespace Web::XHR {
 
+static constexpr Array<u8, 4> http_whitespace_bytes = { '\t', '\n', '\r', ' ' };
+
 class XMLHttpRequest final
     : public RefCounted<XMLHttpRequest>
     , public Weakable<XMLHttpRequest>