diff --git a/Userland/Libraries/LibWeb/CSS/Parser/ComponentValue.cpp b/Userland/Libraries/LibWeb/CSS/Parser/ComponentValue.cpp
index 9b9a4adca4..2505db04e7 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/ComponentValue.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/ComponentValue.cpp
@@ -31,7 +31,7 @@ DeprecatedString ComponentValue::to_deprecated_string() const
     return m_value.visit(
         [](Token const& token) { return token.to_string().release_value_but_fixme_should_propagate_errors().to_deprecated_string(); },
         [](NonnullRefPtr<Block> const& block) { return block->to_string().release_value_but_fixme_should_propagate_errors().to_deprecated_string(); },
-        [](NonnullRefPtr<Function> const& function) { return function->to_deprecated_string(); });
+        [](NonnullRefPtr<Function> const& function) { return function->to_string().release_value_but_fixme_should_propagate_errors().to_deprecated_string(); });
 }
 
 ErrorOr<String> ComponentValue::to_debug_string() const
@@ -44,7 +44,7 @@ ErrorOr<String> ComponentValue::to_debug_string() const
             return String::formatted("Block: {}", TRY(block->to_string()));
         },
         [](NonnullRefPtr<Function> const& function) -> ErrorOr<String> {
-            return String::formatted("Function: {}", function->to_deprecated_string());
+            return String::formatted("Function: {}", TRY(function->to_string()));
         });
 }
 
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Function.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Function.cpp
index 8d4a06c139..e5e8a935ab 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Function.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Function.cpp
@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020-2021, the SerenityOS developers.
- * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2023, Sam Atkins <atkinssj@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -10,7 +10,7 @@
 
 namespace Web::CSS::Parser {
 
-Function::Function(DeprecatedFlyString name, Vector<ComponentValue>&& values)
+Function::Function(FlyString name, Vector<ComponentValue>&& values)
     : m_name(move(name))
     , m_values(move(values))
 {
@@ -18,16 +18,16 @@ Function::Function(DeprecatedFlyString name, Vector<ComponentValue>&& values)
 
 Function::~Function() = default;
 
-DeprecatedString Function::to_deprecated_string() const
+ErrorOr<String> Function::to_string() const
 {
     StringBuilder builder;
 
     serialize_an_identifier(builder, m_name);
-    builder.append('(');
-    builder.join(' ', m_values);
-    builder.append(')');
+    TRY(builder.try_append('('));
+    TRY(builder.try_join(' ', m_values));
+    TRY(builder.try_append(')'));
 
-    return builder.to_deprecated_string();
+    return builder.to_string();
 }
 
 }
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Function.h b/Userland/Libraries/LibWeb/CSS/Parser/Function.h
index 6eb161bbc8..88928b7b49 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Function.h
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Function.h
@@ -1,15 +1,15 @@
 /*
  * Copyright (c) 2020-2021, the SerenityOS developers.
- * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2023, Sam Atkins <atkinssj@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
-#include <AK/DeprecatedFlyString.h>
-#include <AK/DeprecatedString.h>
+#include <AK/FlyString.h>
 #include <AK/RefCounted.h>
+#include <AK/String.h>
 #include <AK/Vector.h>
 #include <LibWeb/CSS/Parser/ComponentValue.h>
 #include <LibWeb/Forward.h>
@@ -18,7 +18,7 @@ namespace Web::CSS::Parser {
 
 class Function : public RefCounted<Function> {
 public:
-    static NonnullRefPtr<Function> create(DeprecatedFlyString name, Vector<ComponentValue>&& values)
+    static NonnullRefPtr<Function> create(FlyString name, Vector<ComponentValue>&& values)
     {
         return adopt_ref(*new Function(move(name), move(values)));
     }
@@ -28,12 +28,12 @@ public:
     StringView name() const { return m_name; }
     Vector<ComponentValue> const& values() const { return m_values; }
 
-    DeprecatedString to_deprecated_string() const;
+    ErrorOr<String> to_string() const;
 
 private:
-    Function(DeprecatedFlyString name, Vector<ComponentValue>&& values);
+    Function(FlyString name, Vector<ComponentValue>&& values);
 
-    DeprecatedFlyString m_name;
+    FlyString m_name;
     Vector<ComponentValue> m_values;
 };
 }
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
index f264d30c24..e433e50ad2 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -1801,7 +1801,7 @@ NonnullRefPtr<Function> Parser::consume_a_function(TokenStream<T>& tokens)
     // Create a function with its name equal to the value of the current input token
     // and with its value initially set to an empty list.
     // NOTE: We create the Function fully initialized when we return it instead.
-    DeprecatedFlyString function_name = ((Token)name_ident).function();
+    auto function_name = FlyString::from_utf8(((Token)name_ident).function()).release_value_but_fixme_should_propagate_errors();
     Vector<ComponentValue> function_values;
 
     // Repeatedly consume the next input token and process it as follows:
diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
index a672475399..4dbb41256a 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -628,7 +628,7 @@ bool StyleComputer::expand_variables(DOM::Element& element, StringView property_
             Parser::TokenStream source_function_contents { source_function.values() };
             if (!expand_variables(element, property_name, dependencies, source_function_contents, function_values))
                 return false;
-            NonnullRefPtr<Parser::Function> function = Parser::Function::create(source_function.name(), move(function_values));
+            NonnullRefPtr<Parser::Function> function = Parser::Function::create(FlyString::from_utf8(source_function.name()).release_value_but_fixme_should_propagate_errors(), move(function_values));
             dest.empend(function);
             continue;
         }
@@ -750,7 +750,8 @@ bool StyleComputer::expand_unresolved_values(DOM::Element& element, StringView p
             Parser::TokenStream source_function_contents { source_function.values() };
             if (!expand_unresolved_values(element, property_name, source_function_contents, function_values))
                 return false;
-            NonnullRefPtr<Parser::Function> function = Parser::Function::create(source_function.name(), move(function_values));
+            // FIXME: This would be much nicer if we could access the source_function's FlyString value directly.
+            NonnullRefPtr<Parser::Function> function = Parser::Function::create(FlyString::from_utf8(source_function.name()).release_value_but_fixme_should_propagate_errors(), move(function_values));
             dest.empend(function);
             continue;
         }