diff --git a/Tests/LibWeb/Text/expected/all-window-properties.txt b/Tests/LibWeb/Text/expected/all-window-properties.txt
index 8c09febb7563..97fe913fcd10 100644
--- a/Tests/LibWeb/Text/expected/all-window-properties.txt
+++ b/Tests/LibWeb/Text/expected/all-window-properties.txt
@@ -63,7 +63,6 @@ Crypto
 CryptoKey
 CustomElementRegistry
 CustomEvent
-DecompressionStream
 DOMException
 DOMImplementation
 DOMMatrix
@@ -83,6 +82,7 @@ DataTransferItem
 DataTransferItemList
 DataView
 Date
+DecompressionStream
 DisposableStack
 Document
 DocumentFragment
diff --git a/Userland/Libraries/LibCompress/Zlib.cpp b/Userland/Libraries/LibCompress/Zlib.cpp
index 6978eae94c00..17d0c3c51ee7 100644
--- a/Userland/Libraries/LibCompress/Zlib.cpp
+++ b/Userland/Libraries/LibCompress/Zlib.cpp
@@ -171,4 +171,20 @@ ErrorOr<ByteBuffer> ZlibCompressor::compress_all(ReadonlyBytes bytes, ZlibCompre
     return buffer;
 }
 
+ErrorOr<ByteBuffer> ZlibDecompressor::decompress_all(ReadonlyBytes bytes)
+{
+    // Even though the content encoding is "deflate", it's actually deflate with the zlib wrapper.
+    // https://tools.ietf.org/html/rfc7230#section-4.2.2
+    auto memory_stream = TRY(try_make<FixedMemoryStream>(bytes));
+    auto zlib_decompressor = ZlibDecompressor::create(move(memory_stream));
+    if (zlib_decompressor.is_error()) {
+        // From the RFC:
+        // "Note: Some non-conformant implementations send the "deflate"
+        //        compressed data without the zlib wrapper."
+        return DeflateDecompressor::decompress_all(bytes);
+    } else {
+        return zlib_decompressor.value()->read_until_eof();
+    }
+}
+
 }
diff --git a/Userland/Libraries/LibCompress/Zlib.h b/Userland/Libraries/LibCompress/Zlib.h
index 478ead7f24ed..20228c455df2 100644
--- a/Userland/Libraries/LibCompress/Zlib.h
+++ b/Userland/Libraries/LibCompress/Zlib.h
@@ -54,6 +54,8 @@ class ZlibDecompressor : public Stream {
     virtual bool is_open() const override;
     virtual void close() override;
 
+    static ErrorOr<ByteBuffer> decompress_all(ReadonlyBytes);
+
 private:
     ZlibDecompressor(ZlibHeader, NonnullOwnPtr<Stream>);
 
@@ -72,7 +74,6 @@ class ZlibCompressor : public Stream {
     virtual bool is_open() const override;
     virtual void close() override;
     ErrorOr<void> finish();
-
     static ErrorOr<ByteBuffer> compress_all(ReadonlyBytes bytes, ZlibCompressionLevel = ZlibCompressionLevel::Default);
 
 private:
diff --git a/Userland/Libraries/LibWeb/Compression/CompressionStream.cpp b/Userland/Libraries/LibWeb/Compression/CompressionStream.cpp
index 0d0ee6ea5542..682a1055b47e 100644
--- a/Userland/Libraries/LibWeb/Compression/CompressionStream.cpp
+++ b/Userland/Libraries/LibWeb/Compression/CompressionStream.cpp
@@ -26,7 +26,28 @@ JS_DEFINE_ALLOCATOR(CompressionStream);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<CompressionStream>> CompressionStream::construct_impl(JS::Realm& realm, Bindings::CompressionFormat format)
 {
-    return realm.heap().allocate<CompressionStream>(realm, realm, format);
+    auto stream = realm.heap().allocate<CompressionStream>(realm, realm, format);
+
+    auto& vm = realm.vm();
+    auto* env = vm.variable_environment();
+    if (env) {
+        // FIXME: Make this private to the web execution context
+        auto& global_object = realm.global_object();
+        auto constructor_value_or_error = global_object.get("CompressionStream_constructor");
+        auto constructor_value = TRY(constructor_value_or_error);
+        if (constructor_value.is_empty() || constructor_value.is_undefined()) {
+            return WebIDL::SimpleException {
+                WebIDL::SimpleExceptionType::TypeError,
+                "CompressionStream constructor not found"sv
+            };
+        }
+
+        auto& func = static_cast<JS::ECMAScriptFunctionObject&>(constructor_value.as_function());
+        JS::MarkedVector<JS::Value> arguments_list { vm.heap() };
+        arguments_list.append(JS::PrimitiveString::create(vm, Bindings::idl_enum_to_string(format)));
+        TRY(func.internal_call(stream->m_this_value, move(arguments_list)));
+    }
+    return stream;
 }
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Uint8Array>> CompressionStream::compress(JS::VM& vm, Bindings::CompressionFormat format, JS::Handle<WebIDL::BufferSource> buffer_source)
@@ -63,7 +84,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Uint8Array>> CompressionStream::compres
 static JS::GCPtr<JS::Script> import_js_script(JS::Realm& realm)
 {
     auto& vm = realm.vm();
-    auto file = MUST(Core::File::open("Userland/Libraries/LibWeb/Compression/CompressionStream.js"sv, Core::File::OpenMode::Read));
+    auto file = MUST(Core::File::open("/Users/johandahlin/dev/ladybird/Userland/Libraries/LibWeb/Compression/CompressionStream.js"sv, Core::File::OpenMode::Read));
     auto file_contents = MUST(file->read_until_eof());
     auto source = StringView { file_contents };
 
@@ -78,16 +99,6 @@ CompressionStream::CompressionStream(JS::Realm& realm, Bindings::CompressionForm
     , m_js_script(import_js_script(realm))
     , m_this_value(JS::Object::create(realm, realm.intrinsics().object_prototype()))
 {
-    auto& vm = realm.vm();
-    auto* env = vm.variable_environment();
-    if (env) {
-        // FIXME: Make this private to the web execution context
-        auto constructor_value = MUST(env->get_binding_value(vm, "CompressionStream_constructor", true));
-        auto& func = static_cast<JS::ECMAScriptFunctionObject&>(constructor_value.as_function());
-        JS::MarkedVector<JS::Value> arguments_list { vm.heap() };
-        arguments_list.append(JS::PrimitiveString::create(vm, Bindings::idl_enum_to_string(format)));
-        MUST(func.internal_call(m_this_value, move(arguments_list)));
-    }
 }
 
 CompressionStream::~CompressionStream() = default;
diff --git a/Userland/Libraries/LibWeb/Compression/DecompressionStream.cpp b/Userland/Libraries/LibWeb/Compression/DecompressionStream.cpp
index 722d35ba2ac4..d93e29604ab4 100644
--- a/Userland/Libraries/LibWeb/Compression/DecompressionStream.cpp
+++ b/Userland/Libraries/LibWeb/Compression/DecompressionStream.cpp
@@ -26,7 +26,28 @@ JS_DEFINE_ALLOCATOR(DecompressionStream);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<DecompressionStream>> DecompressionStream::construct_impl(JS::Realm& realm, Bindings::CompressionFormat format)
 {
-    return realm.heap().allocate<DecompressionStream>(realm, realm, format);
+    auto stream = realm.heap().allocate<DecompressionStream>(realm, realm, format);
+
+    auto& vm = realm.vm();
+    auto* env = vm.variable_environment();
+    if (env) {
+        // FIXME: Make this private to the web execution context
+        auto& global_object = realm.global_object();
+        auto constructor_value_or_error = global_object.get("DecompressionStream_constructor");
+        auto constructor_value = TRY(constructor_value_or_error);
+        if (constructor_value.is_empty() || constructor_value.is_undefined()) {
+            return WebIDL::SimpleException {
+                WebIDL::SimpleExceptionType::TypeError,
+                "DecompressionStream constructor not found"sv
+            };
+        }
+
+        auto& func = static_cast<JS::ECMAScriptFunctionObject&>(constructor_value.as_function());
+        JS::MarkedVector<JS::Value> arguments_list { vm.heap() };
+        arguments_list.append(JS::PrimitiveString::create(vm, Bindings::idl_enum_to_string(format)));
+        TRY(func.internal_call(stream->m_this_value, move(arguments_list)));
+    }
+    return stream;
 }
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Uint8Array>> DecompressionStream::decompress(JS::VM& vm, Bindings::CompressionFormat format, JS::Handle<WebIDL::BufferSource> buffer_source)
@@ -65,7 +86,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<JS::Uint8Array>> DecompressionStream::decom
 static JS::GCPtr<JS::Script> import_js_script(JS::Realm& realm)
 {
     auto& vm = realm.vm();
-    auto file = MUST(Core::File::open("Userland/Libraries/LibWeb/Compression/DecompressionStream.js"sv, Core::File::OpenMode::Read));
+    auto file = MUST(Core::File::open("/Users/johandahlin/dev/ladybird/Userland/Libraries/LibWeb/Compression/DecompressionStream.js"sv, Core::File::OpenMode::Read));
     auto file_contents = MUST(file->read_until_eof());
     auto source = StringView { file_contents };
 
@@ -80,16 +101,6 @@ DecompressionStream::DecompressionStream(JS::Realm& realm, Bindings::Compression
     , m_js_script(import_js_script(realm))
     , m_this_value(JS::Object::create(realm, realm.intrinsics().object_prototype()))
 {
-    auto& vm = realm.vm();
-    auto* env = vm.variable_environment();
-    if (env) {
-        // FIXME: Make this private to the web execution context
-        auto constructor_value = MUST(env->get_binding_value(vm, "DecompressionStream_constructor", true));
-        auto& func = static_cast<JS::ECMAScriptFunctionObject&>(constructor_value.as_function());
-        JS::MarkedVector<JS::Value> arguments_list { vm.heap() };
-        arguments_list.append(JS::PrimitiveString::create(vm, Bindings::idl_enum_to_string(format)));
-        MUST(func.internal_call(m_this_value, move(arguments_list)));
-    }
 }
 
 DecompressionStream::~DecompressionStream() = default;