diff --git a/Libraries/LibWeb/XHR/XMLHttpRequest.cpp b/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
index ae11748bc56..ee96d0ba472 100644
--- a/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
+++ b/Libraries/LibWeb/XHR/XMLHttpRequest.cpp
@@ -972,13 +972,15 @@ void XMLHttpRequest::set_onreadystatechange(WebIDL::CallbackType* value)
 }
 
 // https://xhr.spec.whatwg.org/#dom-xmlhttprequest-getresponseheader
-WebIDL::ExceptionOr<Optional<String>> XMLHttpRequest::get_response_header(String const& name) const
+Optional<String> XMLHttpRequest::get_response_header(String const& name) const
 {
-    auto& vm = this->vm();
-
     // The getResponseHeader(name) method steps are to return the result of getting name from this’s response’s header list.
     auto header_bytes = m_response->header_list()->get(name.bytes());
-    return header_bytes.has_value() ? TRY_OR_THROW_OOM(vm, String::from_utf8(*header_bytes)) : Optional<String> {};
+    if (!header_bytes.has_value())
+        return {};
+
+    // FIXME: The spec doesn't mention isomorphic decode. Spec bug?
+    return Infra::isomorphic_decode(header_bytes->bytes());
 }
 
 // https://xhr.spec.whatwg.org/#legacy-uppercased-byte-less-than
@@ -997,12 +999,10 @@ static ErrorOr<bool> is_legacy_uppercased_byte_less_than(ReadonlyBytes a, Readon
 }
 
 // https://xhr.spec.whatwg.org/#dom-xmlhttprequest-getallresponseheaders
-WebIDL::ExceptionOr<String> XMLHttpRequest::get_all_response_headers() const
+String XMLHttpRequest::get_all_response_headers() const
 {
-    auto& vm = this->vm();
-
     // 1. Let output be an empty byte sequence.
-    ByteBuffer output;
+    StringBuilder output;
 
     // 2. Let initialHeaders be the result of running sort and combine with this’s response’s header list.
     auto initial_headers = m_response->header_list()->sort_and_combine();
@@ -1011,8 +1011,7 @@ WebIDL::ExceptionOr<String> XMLHttpRequest::get_all_response_headers() const
     // Spec Note: Unfortunately, this is needed for compatibility with deployed content.
     // NOTE: quick_sort mutates the collection instead of returning a sorted copy.
     quick_sort(initial_headers, [](Fetch::Infrastructure::Header const& a, Fetch::Infrastructure::Header const& b) {
-        // FIXME: We are not in a context where we can throw from OOM.
-        return is_legacy_uppercased_byte_less_than(a.name, b.name).release_value_but_fixme_should_propagate_errors();
+        return MUST(is_legacy_uppercased_byte_less_than(a.name, b.name));
     });
 
     // 4. For each header in headers, append header’s name, followed by a 0x3A 0x20 byte pair, followed by header’s value, followed by a 0x0D 0x0A byte pair, to output.
@@ -1020,13 +1019,14 @@ WebIDL::ExceptionOr<String> XMLHttpRequest::get_all_response_headers() const
         output.append(header.name);
         output.append(0x3A); // ':'
         output.append(0x20); // ' '
-        output.append(header.value);
+        // FIXME: The spec does not mention isomorphic decode. Spec bug?
+        output.append(Infra::isomorphic_decode(header.value).bytes());
         output.append(0x0D); // '\r'
         output.append(0x0A); // '\n'
     }
 
     // 5. Return output.
-    return TRY_OR_THROW_OOM(vm, String::from_utf8(output));
+    return MUST(output.to_string());
 }
 
 // https://xhr.spec.whatwg.org/#dom-xmlhttprequest-overridemimetype
diff --git a/Libraries/LibWeb/XHR/XMLHttpRequest.h b/Libraries/LibWeb/XHR/XMLHttpRequest.h
index accd6b75968..21d73e492be 100644
--- a/Libraries/LibWeb/XHR/XMLHttpRequest.h
+++ b/Libraries/LibWeb/XHR/XMLHttpRequest.h
@@ -61,8 +61,8 @@ public:
     WebIDL::ExceptionOr<void> set_request_header(String const& header, String const& value);
     WebIDL::ExceptionOr<void> set_response_type(Bindings::XMLHttpRequestResponseType);
 
-    WebIDL::ExceptionOr<Optional<String>> get_response_header(String const& name) const;
-    WebIDL::ExceptionOr<String> get_all_response_headers() const;
+    Optional<String> get_response_header(String const& name) const;
+    String get_all_response_headers() const;
 
     WebIDL::CallbackType* onreadystatechange();
     void set_onreadystatechange(WebIDL::CallbackType*);
diff --git a/Tests/LibWeb/Text/expected/XHR/XMLHttpRequest-response-header-decoding.txt b/Tests/LibWeb/Text/expected/XHR/XMLHttpRequest-response-header-decoding.txt
new file mode 100644
index 00000000000..61eb7fcf0fc
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/XHR/XMLHttpRequest-response-header-decoding.txt
@@ -0,0 +1,4 @@
+getAllResponseHeaders()
+refresh: 0;./refreshed.txt?€ÿ
+
+getResponseHeader("Refresh") => '0;./refreshed.txt?€ÿ'
diff --git a/Tests/LibWeb/Text/input/XHR/XMLHttpRequest-response-header-decoding.html b/Tests/LibWeb/Text/input/XHR/XMLHttpRequest-response-header-decoding.html
new file mode 100644
index 00000000000..f43133a8b7d
--- /dev/null
+++ b/Tests/LibWeb/Text/input/XHR/XMLHttpRequest-response-header-decoding.html
@@ -0,0 +1,29 @@
+<script src="../include.js"></script>
+<script>
+    asyncTest(async (done) => {
+        try {
+            const httpServer = httpTestServer();
+            const url = await httpServer.createEcho("GET", "/xml-http-request-response-header-decoding", {
+                status: 200,
+                headers: {
+                    "Access-Control-Allow-Origin": "*",
+                    "Access-Control-Expose-Headers": "Refresh",
+                    "Refresh": "0;./refreshed.txt?\u0080\u00FF",
+                },
+                body: "",
+            });
+
+            const xhr = new XMLHttpRequest();
+            xhr.open("GET", url);
+
+            xhr.addEventListener("load", () => {
+                println(`getAllResponseHeaders()\n${xhr.getAllResponseHeaders().replace('\r','')}`);
+                println(`getResponseHeader("Refresh") => '${xhr.getResponseHeader("Refresh")}'`);
+                done();
+            });
+            xhr.send();
+        } catch (err) {
+            console.log("FAIL - " + err);
+        }
+    });
+</script>