diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
index c59b711508..b2aa565aa3 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -3714,6 +3714,30 @@ ErrorOr<OwnPtr<CalculationNode>> Parser::parse_acos_function(Function const& fun
     return TRY(AcosCalculationNode::create(calculation_node.release_nonnull()));
 }
 
+ErrorOr<OwnPtr<CalculationNode>> Parser::parse_atan_function(Function const& function)
+{
+    auto calculation_node = TRY(parse_a_calculation(function.values()));
+
+    if (!calculation_node) {
+        dbgln_if(CSS_PARSER_DEBUG, "atan() parameter must be a valid calculation"sv);
+        return nullptr;
+    }
+
+    auto maybe_parameter_type = calculation_node->resolved_type();
+    if (!maybe_parameter_type.has_value()) {
+        dbgln_if(CSS_PARSER_DEBUG, "Failed to resolve type for atan() parameter"sv);
+        return nullptr;
+    }
+
+    auto resolved_type = maybe_parameter_type.value();
+    if (resolved_type != CalculatedStyleValue::ResolvedType::Number) {
+        dbgln_if(CSS_PARSER_DEBUG, "atan() parameter must be a number"sv);
+        return nullptr;
+    }
+
+    return TRY(AtanCalculationNode::create(calculation_node.release_nonnull()));
+}
+
 ErrorOr<RefPtr<StyleValue>> Parser::parse_dynamic_value(ComponentValue const& component_value)
 {
     if (component_value.is_function()) {
@@ -3773,6 +3797,9 @@ ErrorOr<OwnPtr<CalculationNode>> Parser::parse_a_calc_function_node(Function con
     if (function.name().equals_ignoring_ascii_case("acos"sv))
         return TRY(parse_acos_function(function));
 
+    if (function.name().equals_ignoring_ascii_case("atan"sv))
+        return TRY(parse_atan_function(function));
+
     dbgln_if(CSS_PARSER_DEBUG, "We didn't implement `{}` function yet", function.name());
     return nullptr;
 }
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
index 0609fb0495..241a40b983 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
@@ -300,6 +300,7 @@ private:
     ErrorOr<OwnPtr<CalculationNode>> parse_tan_function(Function const&);
     ErrorOr<OwnPtr<CalculationNode>> parse_asin_function(Function const&);
     ErrorOr<OwnPtr<CalculationNode>> parse_acos_function(Function const&);
+    ErrorOr<OwnPtr<CalculationNode>> parse_atan_function(Function const&);
     ErrorOr<RefPtr<StyleValue>> parse_dimension_value(ComponentValue const&);
     ErrorOr<RefPtr<StyleValue>> parse_integer_value(TokenStream<ComponentValue>&);
     ErrorOr<RefPtr<StyleValue>> parse_number_value(TokenStream<ComponentValue>&);
diff --git a/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.cpp b/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.cpp
index 9beba5b0ca..e8428a3fee 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.cpp
@@ -1120,6 +1120,60 @@ ErrorOr<void> AcosCalculationNode::dump(StringBuilder& builder, int indent) cons
     return {};
 }
 
+ErrorOr<NonnullOwnPtr<AtanCalculationNode>> AtanCalculationNode::create(NonnullOwnPtr<CalculationNode> value)
+{
+    return adopt_nonnull_own_or_enomem(new (nothrow) AtanCalculationNode(move(value)));
+}
+
+AtanCalculationNode::AtanCalculationNode(NonnullOwnPtr<CalculationNode> value)
+    : CalculationNode(Type::Atan)
+    , m_value(move(value))
+{
+}
+
+AtanCalculationNode::~AtanCalculationNode() = default;
+
+ErrorOr<String> AtanCalculationNode::to_string() const
+{
+    StringBuilder builder;
+    builder.append("atan("sv);
+    builder.append(TRY(m_value->to_string()));
+    builder.append(")"sv);
+    return builder.to_string();
+}
+
+Optional<CalculatedStyleValue::ResolvedType> AtanCalculationNode::resolved_type() const
+{
+    return CalculatedStyleValue::ResolvedType::Angle;
+}
+
+bool AtanCalculationNode::contains_percentage() const
+{
+    return m_value->contains_percentage();
+}
+
+CalculatedStyleValue::CalculationResult AtanCalculationNode::resolve(Optional<Length::ResolutionContext const&> context, CalculatedStyleValue::PercentageBasis const& percentage_basis) const
+{
+    auto node_a = m_value->resolve(context, percentage_basis);
+    auto node_a_value = resolve_value(node_a.value(), context);
+    auto result = atan(node_a_value);
+
+    return { Angle(result, Angle::Type::Rad) };
+}
+
+ErrorOr<void> AtanCalculationNode::for_each_child_node(Function<ErrorOr<void>(NonnullOwnPtr<CalculationNode>&)> const& callback)
+{
+    TRY(m_value->for_each_child_node(callback));
+    TRY(callback(m_value));
+    return {};
+}
+
+ErrorOr<void> AtanCalculationNode::dump(StringBuilder& builder, int indent) const
+{
+    TRY(builder.try_appendff("{: >{}}ATAN: {}\n", "", indent, TRY(to_string())));
+    return {};
+}
+
 void CalculatedStyleValue::CalculationResult::add(CalculationResult const& other, Optional<Length::ResolutionContext const&> context, PercentageBasis const& percentage_basis)
 {
     add_or_subtract_internal(SumOperation::Add, other, context, percentage_basis);
diff --git a/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.h b/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.h
index 7c84f38b07..43f2c732c3 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.h
+++ b/Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.h
@@ -486,4 +486,22 @@ private:
     NonnullOwnPtr<CalculationNode> m_value;
 };
 
+class AtanCalculationNode final : public CalculationNode {
+public:
+    static ErrorOr<NonnullOwnPtr<AtanCalculationNode>> create(NonnullOwnPtr<CalculationNode>);
+    ~AtanCalculationNode();
+
+    virtual ErrorOr<String> to_string() const override;
+    virtual Optional<CalculatedStyleValue::ResolvedType> resolved_type() const override;
+    virtual bool contains_percentage() const override;
+    virtual CalculatedStyleValue::CalculationResult resolve(Optional<Length::ResolutionContext const&>, CalculatedStyleValue::PercentageBasis const&) const override;
+    virtual ErrorOr<void> for_each_child_node(Function<ErrorOr<void>(NonnullOwnPtr<CalculationNode>&)> const&) override;
+
+    virtual ErrorOr<void> dump(StringBuilder&, int indent) const override;
+
+private:
+    AtanCalculationNode(NonnullOwnPtr<CalculationNode>);
+    NonnullOwnPtr<CalculationNode> m_value;
+};
+
 }