diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index 5c083401c0..8665b6e697 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -237,6 +237,7 @@ set(SOURCES
     HTML/WorkerLocation.cpp
     HighResolutionTime/Performance.cpp
     ImageDecoding.cpp
+    Infra/ByteSequences.cpp
     IntersectionObserver/IntersectionObserver.cpp
     Layout/BlockContainer.cpp
     Layout/BlockFormattingContext.cpp
diff --git a/Userland/Libraries/LibWeb/Infra/ByteSequences.cpp b/Userland/Libraries/LibWeb/Infra/ByteSequences.cpp
new file mode 100644
index 0000000000..f54008c0db
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Infra/ByteSequences.cpp
@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/CharacterTypes.h>
+#include <LibWeb/Infra/ByteSequences.h>
+
+namespace Web::Infra {
+
+// https://infra.spec.whatwg.org/#byte-lowercase
+void byte_lowercase(ByteBuffer& bytes)
+{
+    // To byte-lowercase a byte sequence, increase each byte it contains, in the range 0x41 (A) to 0x5A (Z), inclusive, by 0x20.
+    for (size_t i = 0; i < bytes.size(); ++i)
+        bytes[i] = to_ascii_lowercase(bytes[i]);
+}
+
+// https://infra.spec.whatwg.org/#byte-uppercase
+void byte_uppercase(ByteBuffer& bytes)
+{
+    // To byte-uppercase a byte sequence, subtract each byte it contains, in the range 0x61 (a) to 0x7A (z), inclusive, by 0x20.
+    for (size_t i = 0; i < bytes.size(); ++i)
+        bytes[i] = to_ascii_uppercase(bytes[i]);
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/Infra/ByteSequences.h b/Userland/Libraries/LibWeb/Infra/ByteSequences.h
new file mode 100644
index 0000000000..aac82e314e
--- /dev/null
+++ b/Userland/Libraries/LibWeb/Infra/ByteSequences.h
@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+
+namespace Web::Infra {
+
+void byte_lowercase(ByteBuffer&);
+void byte_uppercase(ByteBuffer&);
+
+}