diff --git a/Libraries/LibWeb/CMakeLists.txt b/Libraries/LibWeb/CMakeLists.txt
index 5f9d1cac8b5..5628ae9a08e 100644
--- a/Libraries/LibWeb/CMakeLists.txt
+++ b/Libraries/LibWeb/CMakeLists.txt
@@ -116,6 +116,7 @@ set(SOURCES
     CSS/CSSLayerBlockRule.cpp
     CSS/CSSLayerStatementRule.cpp
     CSS/CSSMarginRule.cpp
+    CSS/CSSMathInvert.cpp
     CSS/CSSMathNegate.cpp
     CSS/CSSMathProduct.cpp
     CSS/CSSMathSum.cpp
diff --git a/Libraries/LibWeb/CSS/CSSMathInvert.cpp b/Libraries/LibWeb/CSS/CSSMathInvert.cpp
new file mode 100644
index 00000000000..62493796ed0
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathInvert.cpp
@@ -0,0 +1,95 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSMathInvert.h"
+#include <LibWeb/Bindings/CSSMathInvertPrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::CSS {
+
+GC_DEFINE_ALLOCATOR(CSSMathInvert);
+
+GC::Ref<CSSMathInvert> CSSMathInvert::create(JS::Realm& realm, NumericType type, GC::Ref<CSSNumericValue> values)
+{
+    return realm.create<CSSMathInvert>(realm, move(type), move(values));
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssmathinvert-cssmathinvert
+WebIDL::ExceptionOr<GC::Ref<CSSMathInvert>> CSSMathInvert::construct_impl(JS::Realm& realm, CSSNumberish value)
+{
+    // The CSSMathInvert(arg) constructor is defined identically to the above, except that in the last step it returns
+    // a new CSSMathInvert object.
+    // NB: So, the steps below are a modification of the CSSMathNegate steps.
+
+    // 1. Replace arg with the result of rectifying a numberish value for arg.
+    auto converted_value = rectify_a_numberish_value(realm, value);
+
+    // 2. Return a new CSSMathInvert whose value internal slot is set to arg.
+    return CSSMathInvert::create(realm, converted_value->type().inverted(), converted_value);
+}
+
+CSSMathInvert::CSSMathInvert(JS::Realm& realm, NumericType type, GC::Ref<CSSNumericValue> values)
+    : CSSMathValue(realm, Bindings::CSSMathOperator::Invert, move(type))
+    , m_value(move(values))
+{
+}
+
+CSSMathInvert::~CSSMathInvert() = default;
+
+void CSSMathInvert::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSMathInvert);
+    Base::initialize(realm);
+}
+
+void CSSMathInvert::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_value);
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
+String CSSMathInvert::serialize_math_value(Nested nested, Parens parens) const
+{
+    // NB: Only steps 1 and 6 apply here.
+    // 1. Let s initially be the empty string.
+    StringBuilder s;
+
+    // 6. Otherwise, if this is a CSSMathInvert:
+    {
+        // 1. If paren-less is true, continue to the next step; otherwise, if nested is true, append "(" to s;
+        //    otherwise, append "calc(" to s.
+        if (parens == Parens::With) {
+            if (nested == Nested::Yes) {
+                s.append("("sv);
+            } else {
+                s.append("calc("sv);
+            }
+        }
+
+        // 2. Append "1 / " to s.
+        s.append("1 / "sv);
+
+        // 3. Serialize this’s value internal slot with nested set to true, and append the result to s.
+        s.append(m_value->to_string({ .nested = true }));
+
+        // 4. If paren-less is false, append ")" to s,
+        if (parens == Parens::With)
+            s.append(")"sv);
+
+        // 5. Return s.
+        return s.to_string_without_validation();
+    }
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssmathinvert-value
+GC::Ref<CSSNumericValue> CSSMathInvert::value() const
+{
+    return m_value;
+}
+
+}
diff --git a/Libraries/LibWeb/CSS/CSSMathInvert.h b/Libraries/LibWeb/CSS/CSSMathInvert.h
new file mode 100644
index 00000000000..231bd14af19
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathInvert.h
@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSMathValue.h>
+
+namespace Web::CSS {
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathinvert
+class CSSMathInvert : public CSSMathValue {
+    WEB_PLATFORM_OBJECT(CSSMathInvert, CSSMathValue);
+    GC_DECLARE_ALLOCATOR(CSSMathInvert);
+
+public:
+    [[nodiscard]] static GC::Ref<CSSMathInvert> create(JS::Realm&, NumericType, GC::Ref<CSSNumericValue>);
+    static WebIDL::ExceptionOr<GC::Ref<CSSMathInvert>> construct_impl(JS::Realm&, CSSNumberish);
+
+    virtual ~CSSMathInvert() override;
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Visitor&) override;
+
+    GC::Ref<CSSNumericValue> value() const;
+
+    virtual String serialize_math_value(Nested, Parens) const override;
+
+private:
+    CSSMathInvert(JS::Realm&, NumericType, GC::Ref<CSSNumericValue>);
+    GC::Ref<CSSNumericValue> m_value;
+};
+
+}
diff --git a/Libraries/LibWeb/CSS/CSSMathInvert.idl b/Libraries/LibWeb/CSS/CSSMathInvert.idl
new file mode 100644
index 00000000000..a51b6f30f55
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathInvert.idl
@@ -0,0 +1,9 @@
+#import <CSS/CSSMathValue.idl>
+#import <CSS/CSSNumericValue.idl>
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathinvert
+[Exposed=(Window, Worker, PaintWorklet, LayoutWorklet)]
+interface CSSMathInvert : CSSMathValue {
+    constructor(CSSNumberish arg);
+    readonly attribute CSSNumericValue value;
+};
diff --git a/Libraries/LibWeb/CSS/CSSMathProduct.cpp b/Libraries/LibWeb/CSS/CSSMathProduct.cpp
index e94b603a71e..120db0ed6bd 100644
--- a/Libraries/LibWeb/CSS/CSSMathProduct.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathProduct.cpp
@@ -7,6 +7,7 @@
 #include "CSSMathProduct.h"
 #include <LibWeb/Bindings/CSSMathProductPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/CSSMathInvert.h>
 #include <LibWeb/CSS/CSSNumericArray.h>
 #include <LibWeb/WebIDL/DOMException.h>
 #include <LibWeb/WebIDL/ExceptionOr.h>
@@ -110,10 +111,9 @@ String CSSMathProduct::serialize_math_value(Nested nested, Parens parens) const
 
             // 1. If arg is a CSSMathInvert, append " / " to s, then serialize arg’s value internal slot with nested
             //    set to true, and append the result to s.
-            // FIXME: Detect CSSMathInvert once that's implemented.
-            if (false) {
+            if (auto* invert = as_if<CSSMathInvert>(*arg)) {
                 s.append(" / "sv);
-                s.append(arg->to_string({ .nested = true }));
+                s.append(invert->value()->to_string({ .nested = true }));
             }
 
             // 2. Otherwise, append " * " to s, then serialize arg with nested set to true, and append the result to s.
diff --git a/Libraries/LibWeb/Forward.h b/Libraries/LibWeb/Forward.h
index 69ff65e3c1b..1f90a407d04 100644
--- a/Libraries/LibWeb/Forward.h
+++ b/Libraries/LibWeb/Forward.h
@@ -242,6 +242,7 @@ class CSSKeywordValue;
 class CSSLayerBlockRule;
 class CSSLayerStatementRule;
 class CSSMarginRule;
+class CSSMathInvert;
 class CSSMathNegate;
 class CSSMathProduct;
 class CSSMathSum;
diff --git a/Libraries/LibWeb/idl_files.cmake b/Libraries/LibWeb/idl_files.cmake
index 0ba14faa6c0..65bcfeaed12 100644
--- a/Libraries/LibWeb/idl_files.cmake
+++ b/Libraries/LibWeb/idl_files.cmake
@@ -36,6 +36,7 @@ libweb_js_bindings(CSS/CSSKeywordValue)
 libweb_js_bindings(CSS/CSSLayerBlockRule)
 libweb_js_bindings(CSS/CSSLayerStatementRule)
 libweb_js_bindings(CSS/CSSMarginRule)
+libweb_js_bindings(CSS/CSSMathInvert)
 libweb_js_bindings(CSS/CSSMathNegate)
 libweb_js_bindings(CSS/CSSMathProduct)
 libweb_js_bindings(CSS/CSSMathSum)
diff --git a/Tests/LibWeb/Text/expected/all-window-properties.txt b/Tests/LibWeb/Text/expected/all-window-properties.txt
index 95fb9230bf1..7c8aaae834a 100644
--- a/Tests/LibWeb/Text/expected/all-window-properties.txt
+++ b/Tests/LibWeb/Text/expected/all-window-properties.txt
@@ -49,6 +49,7 @@ CSSKeywordValue
 CSSLayerBlockRule
 CSSLayerStatementRule
 CSSMarginRule
+CSSMathInvert
 CSSMathNegate
 CSSMathProduct
 CSSMathSum