diff --git a/Userland/Libraries/LibWeb/Streams/WritableStream.cpp b/Userland/Libraries/LibWeb/Streams/WritableStream.cpp
index 76449faf7a..1775a85893 100644
--- a/Userland/Libraries/LibWeb/Streams/WritableStream.cpp
+++ b/Userland/Libraries/LibWeb/Streams/WritableStream.cpp
@@ -17,7 +17,7 @@
 namespace Web::Streams {
 
 // https://streams.spec.whatwg.org/#ws-constructor
-WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStream>> WritableStream::construct_impl(JS::Realm& realm, Optional<JS::Handle<JS::Object>> const& underlying_sink_object)
+WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStream>> WritableStream::construct_impl(JS::Realm& realm, Optional<JS::Handle<JS::Object>> const& underlying_sink_object, QueuingStrategy const& strategy)
 {
     auto& vm = realm.vm();
 
@@ -36,11 +36,11 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStream>> WritableStream::construct_
     // 4. Perform ! InitializeWritableStream(this).
     // Note: This AO configures slot values which are already specified in the class's field initializers.
 
-    // FIXME: 5. Let sizeAlgorithm be ! ExtractSizeAlgorithm(strategy).
-    SizeAlgorithm size_algorithm = [](auto const&) { return JS::normal_completion(JS::Value(1)); };
+    // 5. Let sizeAlgorithm be ! ExtractSizeAlgorithm(strategy).
+    auto size_algorithm = extract_size_algorithm(strategy);
 
-    // FIXME: 6. Let highWaterMark be ? ExtractHighWaterMark(strategy, 1).
-    auto high_water_mark = 1.0;
+    // 6. Let highWaterMark be ? ExtractHighWaterMark(strategy, 1).
+    auto high_water_mark = TRY(extract_high_water_mark(strategy, 1));
 
     // 7. Perform ? SetUpWritableStreamDefaultControllerFromUnderlyingSink(this, underlyingSink, underlyingSinkDict, highWaterMark, sizeAlgorithm).
     TRY(set_up_writable_stream_default_controller_from_underlying_sink(*writable_stream, underlying_sink, underlying_sink_dict, high_water_mark, move(size_algorithm)));
diff --git a/Userland/Libraries/LibWeb/Streams/WritableStream.h b/Userland/Libraries/LibWeb/Streams/WritableStream.h
index 9e02e9a382..43b9eef25a 100644
--- a/Userland/Libraries/LibWeb/Streams/WritableStream.h
+++ b/Userland/Libraries/LibWeb/Streams/WritableStream.h
@@ -11,6 +11,7 @@
 #include <LibJS/Forward.h>
 #include <LibWeb/Bindings/PlatformObject.h>
 #include <LibWeb/Forward.h>
+#include <LibWeb/Streams/QueuingStrategy.h>
 #include <LibWeb/WebIDL/Promise.h>
 
 namespace Web::Streams {
@@ -42,7 +43,7 @@ public:
         Errored,
     };
 
-    static WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStream>> construct_impl(JS::Realm& realm, Optional<JS::Handle<JS::Object>> const& underlying_sink);
+    static WebIDL::ExceptionOr<JS::NonnullGCPtr<WritableStream>> construct_impl(JS::Realm& realm, Optional<JS::Handle<JS::Object>> const& underlying_sink, QueuingStrategy const& = {});
 
     virtual ~WritableStream() = default;
 
diff --git a/Userland/Libraries/LibWeb/Streams/WritableStream.idl b/Userland/Libraries/LibWeb/Streams/WritableStream.idl
index 19977cfb85..f445e24cc8 100644
--- a/Userland/Libraries/LibWeb/Streams/WritableStream.idl
+++ b/Userland/Libraries/LibWeb/Streams/WritableStream.idl
@@ -1,9 +1,9 @@
+#import <Streams/QueuingStrategy.idl>
 #import <Streams/WritableStreamDefaultWriter.idl>
 
 [Exposed=*, Transferable]
 interface WritableStream {
-    // FIXME: optional QueuingStrategy strategy = {}
-    constructor(optional object underlyingSink);
+    constructor(optional object underlyingSink, optional QueuingStrategy strategy = {});
 
     readonly attribute boolean locked;