diff --git a/Libraries/LibWeb/CSS/Interpolation.cpp b/Libraries/LibWeb/CSS/Interpolation.cpp
index de84c8345d8..282c2795c6f 100644
--- a/Libraries/LibWeb/CSS/Interpolation.cpp
+++ b/Libraries/LibWeb/CSS/Interpolation.cpp
@@ -586,8 +586,13 @@ NonnullRefPtr<CSSStyleValue const> interpolate_value(DOM::Element& element, Calc
             layout_node = *node;
         return CSSColorValue::create_from_color(interpolate_color(from.to_color(layout_node), to.to_color(layout_node), delta), ColorSyntax::Modern);
     }
-    case CSSStyleValue::Type::Integer:
-        return IntegerStyleValue::create(interpolate_raw(from.as_integer().integer(), to.as_integer().integer(), delta));
+    case CSSStyleValue::Type::Integer: {
+        // https://drafts.csswg.org/css-values/#combine-integers
+        // Interpolation of <integer> is defined as Vresult = round((1 - p) × VA + p × VB);
+        // that is, interpolation happens in the real number space as for <number>s, and the result is converted to an <integer> by rounding to the nearest integer.
+        auto interpolated_value = interpolate_raw(from.as_integer().value(), to.as_integer().value(), delta);
+        return IntegerStyleValue::create(round_to<i64>(interpolated_value));
+    }
     case CSSStyleValue::Type::Length: {
         // FIXME: Absolutize values
         auto const& from_length = from.as_length().length();
diff --git a/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/animations/z-index-interpolation.txt b/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/animations/z-index-interpolation.txt
new file mode 100644
index 00000000000..c434b34d0ce
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/css/css-transitions/animations/z-index-interpolation.txt
@@ -0,0 +1,256 @@
+Harness status: OK
+
+Found 250 tests
+
+170 Pass
+80 Fail
+Fail	CSS Transitions: property <z-index> from neutral to [5] at (-0.3) should be [-4]
+Fail	CSS Transitions: property <z-index> from neutral to [5] at (0) should be [-2]
+Fail	CSS Transitions: property <z-index> from neutral to [5] at (0.3) should be [0]
+Fail	CSS Transitions: property <z-index> from neutral to [5] at (0.6) should be [2]
+Pass	CSS Transitions: property <z-index> from neutral to [5] at (1) should be [5]
+Fail	CSS Transitions: property <z-index> from neutral to [5] at (1.5) should be [9]
+Fail	CSS Transitions with transition: all: property <z-index> from neutral to [5] at (-0.3) should be [-4]
+Fail	CSS Transitions with transition: all: property <z-index> from neutral to [5] at (0) should be [-2]
+Fail	CSS Transitions with transition: all: property <z-index> from neutral to [5] at (0.3) should be [0]
+Fail	CSS Transitions with transition: all: property <z-index> from neutral to [5] at (0.6) should be [2]
+Pass	CSS Transitions with transition: all: property <z-index> from neutral to [5] at (1) should be [5]
+Fail	CSS Transitions with transition: all: property <z-index> from neutral to [5] at (1.5) should be [9]
+Fail	CSS Animations: property <z-index> from neutral to [5] at (-0.3) should be [-4]
+Fail	CSS Animations: property <z-index> from neutral to [5] at (0) should be [-2]
+Fail	CSS Animations: property <z-index> from neutral to [5] at (0.3) should be [0]
+Fail	CSS Animations: property <z-index> from neutral to [5] at (0.6) should be [2]
+Pass	CSS Animations: property <z-index> from neutral to [5] at (1) should be [5]
+Fail	CSS Animations: property <z-index> from neutral to [5] at (1.5) should be [9]
+Fail	Web Animations: property <z-index> from neutral to [5] at (-0.3) should be [-4]
+Fail	Web Animations: property <z-index> from neutral to [5] at (0) should be [-2]
+Fail	Web Animations: property <z-index> from neutral to [5] at (0.3) should be [0]
+Fail	Web Animations: property <z-index> from neutral to [5] at (0.6) should be [2]
+Pass	Web Animations: property <z-index> from neutral to [5] at (1) should be [5]
+Fail	Web Animations: property <z-index> from neutral to [5] at (1.5) should be [9]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (-0.3) should be [initial]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (0) should be [initial]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (0.3) should be [initial]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (0.5) should be [5]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (0.6) should be [5]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (1) should be [5]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [initial] to [5] at (1.5) should be [5]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (-0.3) should be [initial]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (0) should be [initial]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (0.3) should be [initial]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (0.5) should be [5]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (0.6) should be [5]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (1) should be [5]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [initial] to [5] at (1.5) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (-0.3) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (0) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (0.3) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (0.5) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (0.6) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (1) should be [5]
+Pass	CSS Transitions: property <z-index> from [initial] to [5] at (1.5) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (-0.3) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (0) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (0.3) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (0.5) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (0.6) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (1) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [initial] to [5] at (1.5) should be [5]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (-0.3) should be [initial]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (0) should be [initial]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (0.3) should be [initial]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (0.5) should be [5]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (0.6) should be [5]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (1) should be [5]
+Pass	CSS Animations: property <z-index> from [initial] to [5] at (1.5) should be [5]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (-0.3) should be [initial]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (0) should be [initial]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (0.3) should be [initial]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (0.5) should be [5]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (0.6) should be [5]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (1) should be [5]
+Pass	Web Animations: property <z-index> from [initial] to [5] at (1.5) should be [5]
+Fail	CSS Transitions: property <z-index> from [inherit] to [5] at (-0.3) should be [18]
+Fail	CSS Transitions: property <z-index> from [inherit] to [5] at (0) should be [15]
+Fail	CSS Transitions: property <z-index> from [inherit] to [5] at (0.3) should be [12]
+Fail	CSS Transitions: property <z-index> from [inherit] to [5] at (0.6) should be [9]
+Pass	CSS Transitions: property <z-index> from [inherit] to [5] at (1) should be [5]
+Fail	CSS Transitions: property <z-index> from [inherit] to [5] at (1.5) should be [0]
+Fail	CSS Transitions with transition: all: property <z-index> from [inherit] to [5] at (-0.3) should be [18]
+Fail	CSS Transitions with transition: all: property <z-index> from [inherit] to [5] at (0) should be [15]
+Fail	CSS Transitions with transition: all: property <z-index> from [inherit] to [5] at (0.3) should be [12]
+Fail	CSS Transitions with transition: all: property <z-index> from [inherit] to [5] at (0.6) should be [9]
+Pass	CSS Transitions with transition: all: property <z-index> from [inherit] to [5] at (1) should be [5]
+Fail	CSS Transitions with transition: all: property <z-index> from [inherit] to [5] at (1.5) should be [0]
+Pass	CSS Animations: property <z-index> from [inherit] to [5] at (-0.3) should be [18]
+Pass	CSS Animations: property <z-index> from [inherit] to [5] at (0) should be [15]
+Pass	CSS Animations: property <z-index> from [inherit] to [5] at (0.3) should be [12]
+Pass	CSS Animations: property <z-index> from [inherit] to [5] at (0.6) should be [9]
+Pass	CSS Animations: property <z-index> from [inherit] to [5] at (1) should be [5]
+Pass	CSS Animations: property <z-index> from [inherit] to [5] at (1.5) should be [0]
+Pass	Web Animations: property <z-index> from [inherit] to [5] at (-0.3) should be [18]
+Pass	Web Animations: property <z-index> from [inherit] to [5] at (0) should be [15]
+Pass	Web Animations: property <z-index> from [inherit] to [5] at (0.3) should be [12]
+Pass	Web Animations: property <z-index> from [inherit] to [5] at (0.6) should be [9]
+Pass	Web Animations: property <z-index> from [inherit] to [5] at (1) should be [5]
+Pass	Web Animations: property <z-index> from [inherit] to [5] at (1.5) should be [0]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (-0.3) should be [unset]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (0) should be [unset]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (0.3) should be [unset]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (0.5) should be [5]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (0.6) should be [5]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (1) should be [5]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [unset] to [5] at (1.5) should be [5]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (-0.3) should be [unset]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (0) should be [unset]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (0.3) should be [unset]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (0.5) should be [5]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (0.6) should be [5]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (1) should be [5]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [unset] to [5] at (1.5) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (-0.3) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (0) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (0.3) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (0.5) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (0.6) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (1) should be [5]
+Pass	CSS Transitions: property <z-index> from [unset] to [5] at (1.5) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (-0.3) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (0) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (0.3) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (0.5) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (0.6) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (1) should be [5]
+Pass	CSS Transitions with transition: all: property <z-index> from [unset] to [5] at (1.5) should be [5]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (-0.3) should be [unset]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (0) should be [unset]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (0.3) should be [unset]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (0.5) should be [5]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (0.6) should be [5]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (1) should be [5]
+Pass	CSS Animations: property <z-index> from [unset] to [5] at (1.5) should be [5]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (-0.3) should be [unset]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (0) should be [unset]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (0.3) should be [unset]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (0.5) should be [5]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (0.6) should be [5]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (1) should be [5]
+Pass	Web Animations: property <z-index> from [unset] to [5] at (1.5) should be [5]
+Fail	CSS Transitions: property <z-index> from [-5] to [5] at (-0.3) should be [-8]
+Fail	CSS Transitions: property <z-index> from [-5] to [5] at (0) should be [-5]
+Fail	CSS Transitions: property <z-index> from [-5] to [5] at (0.3) should be [-2]
+Fail	CSS Transitions: property <z-index> from [-5] to [5] at (0.6) should be [1]
+Pass	CSS Transitions: property <z-index> from [-5] to [5] at (1) should be [5]
+Fail	CSS Transitions: property <z-index> from [-5] to [5] at (1.5) should be [10]
+Fail	CSS Transitions with transition: all: property <z-index> from [-5] to [5] at (-0.3) should be [-8]
+Fail	CSS Transitions with transition: all: property <z-index> from [-5] to [5] at (0) should be [-5]
+Fail	CSS Transitions with transition: all: property <z-index> from [-5] to [5] at (0.3) should be [-2]
+Fail	CSS Transitions with transition: all: property <z-index> from [-5] to [5] at (0.6) should be [1]
+Pass	CSS Transitions with transition: all: property <z-index> from [-5] to [5] at (1) should be [5]
+Fail	CSS Transitions with transition: all: property <z-index> from [-5] to [5] at (1.5) should be [10]
+Pass	CSS Animations: property <z-index> from [-5] to [5] at (-0.3) should be [-8]
+Pass	CSS Animations: property <z-index> from [-5] to [5] at (0) should be [-5]
+Pass	CSS Animations: property <z-index> from [-5] to [5] at (0.3) should be [-2]
+Pass	CSS Animations: property <z-index> from [-5] to [5] at (0.6) should be [1]
+Pass	CSS Animations: property <z-index> from [-5] to [5] at (1) should be [5]
+Pass	CSS Animations: property <z-index> from [-5] to [5] at (1.5) should be [10]
+Pass	Web Animations: property <z-index> from [-5] to [5] at (-0.3) should be [-8]
+Pass	Web Animations: property <z-index> from [-5] to [5] at (0) should be [-5]
+Pass	Web Animations: property <z-index> from [-5] to [5] at (0.3) should be [-2]
+Pass	Web Animations: property <z-index> from [-5] to [5] at (0.6) should be [1]
+Pass	Web Animations: property <z-index> from [-5] to [5] at (1) should be [5]
+Pass	Web Animations: property <z-index> from [-5] to [5] at (1.5) should be [10]
+Fail	CSS Transitions: property <z-index> from [2] to [4] at (-0.3) should be [1]
+Fail	CSS Transitions: property <z-index> from [2] to [4] at (0) should be [2]
+Fail	CSS Transitions: property <z-index> from [2] to [4] at (0.3) should be [3]
+Fail	CSS Transitions: property <z-index> from [2] to [4] at (0.6) should be [3]
+Pass	CSS Transitions: property <z-index> from [2] to [4] at (1) should be [4]
+Fail	CSS Transitions: property <z-index> from [2] to [4] at (1.5) should be [5]
+Fail	CSS Transitions with transition: all: property <z-index> from [2] to [4] at (-0.3) should be [1]
+Fail	CSS Transitions with transition: all: property <z-index> from [2] to [4] at (0) should be [2]
+Fail	CSS Transitions with transition: all: property <z-index> from [2] to [4] at (0.3) should be [3]
+Fail	CSS Transitions with transition: all: property <z-index> from [2] to [4] at (0.6) should be [3]
+Pass	CSS Transitions with transition: all: property <z-index> from [2] to [4] at (1) should be [4]
+Fail	CSS Transitions with transition: all: property <z-index> from [2] to [4] at (1.5) should be [5]
+Pass	CSS Animations: property <z-index> from [2] to [4] at (-0.3) should be [1]
+Pass	CSS Animations: property <z-index> from [2] to [4] at (0) should be [2]
+Pass	CSS Animations: property <z-index> from [2] to [4] at (0.3) should be [3]
+Pass	CSS Animations: property <z-index> from [2] to [4] at (0.6) should be [3]
+Pass	CSS Animations: property <z-index> from [2] to [4] at (1) should be [4]
+Pass	CSS Animations: property <z-index> from [2] to [4] at (1.5) should be [5]
+Pass	Web Animations: property <z-index> from [2] to [4] at (-0.3) should be [1]
+Pass	Web Animations: property <z-index> from [2] to [4] at (0) should be [2]
+Pass	Web Animations: property <z-index> from [2] to [4] at (0.3) should be [3]
+Pass	Web Animations: property <z-index> from [2] to [4] at (0.6) should be [3]
+Pass	Web Animations: property <z-index> from [2] to [4] at (1) should be [4]
+Pass	Web Animations: property <z-index> from [2] to [4] at (1.5) should be [5]
+Fail	CSS Transitions: property <z-index> from [-2] to [-4] at (-0.3) should be [-1]
+Fail	CSS Transitions: property <z-index> from [-2] to [-4] at (0) should be [-2]
+Fail	CSS Transitions: property <z-index> from [-2] to [-4] at (0.1) should be [-2]
+Fail	CSS Transitions: property <z-index> from [-2] to [-4] at (0.3) should be [-3]
+Fail	CSS Transitions: property <z-index> from [-2] to [-4] at (0.6) should be [-3]
+Pass	CSS Transitions: property <z-index> from [-2] to [-4] at (1) should be [-4]
+Fail	CSS Transitions: property <z-index> from [-2] to [-4] at (1.5) should be [-5]
+Fail	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (-0.3) should be [-1]
+Fail	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (0) should be [-2]
+Fail	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (0.1) should be [-2]
+Fail	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (0.3) should be [-3]
+Fail	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (0.6) should be [-3]
+Pass	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (1) should be [-4]
+Fail	CSS Transitions with transition: all: property <z-index> from [-2] to [-4] at (1.5) should be [-5]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (-0.3) should be [-1]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (0) should be [-2]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (0.1) should be [-2]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (0.3) should be [-3]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (0.6) should be [-3]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (1) should be [-4]
+Pass	CSS Animations: property <z-index> from [-2] to [-4] at (1.5) should be [-5]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (-0.3) should be [-1]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (0) should be [-2]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (0.1) should be [-2]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (0.3) should be [-3]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (0.6) should be [-3]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (1) should be [-4]
+Pass	Web Animations: property <z-index> from [-2] to [-4] at (1.5) should be [-5]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (-0.3) should be [auto]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (0) should be [auto]
+Fail	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (0.3) should be [auto]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (0.5) should be [10]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (0.6) should be [10]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (1) should be [10]
+Pass	CSS Transitions with transition-behavior:allow-discrete: property <z-index> from [auto] to [10] at (1.5) should be [10]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (-0.3) should be [auto]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (0) should be [auto]
+Fail	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (0.3) should be [auto]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (0.5) should be [10]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (0.6) should be [10]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (1) should be [10]
+Pass	CSS Transitions with transition-property:all and transition-behavor:allow-discrete: property <z-index> from [auto] to [10] at (1.5) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (-0.3) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (0) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (0.3) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (0.5) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (0.6) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (1) should be [10]
+Pass	CSS Transitions: property <z-index> from [auto] to [10] at (1.5) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (-0.3) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (0) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (0.3) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (0.5) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (0.6) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (1) should be [10]
+Pass	CSS Transitions with transition: all: property <z-index> from [auto] to [10] at (1.5) should be [10]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (-0.3) should be [auto]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (0) should be [auto]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (0.3) should be [auto]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (0.5) should be [10]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (0.6) should be [10]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (1) should be [10]
+Pass	CSS Animations: property <z-index> from [auto] to [10] at (1.5) should be [10]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (-0.3) should be [auto]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (0) should be [auto]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (0.3) should be [auto]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (0.5) should be [10]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (0.6) should be [10]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (1) should be [10]
+Pass	Web Animations: property <z-index> from [auto] to [10] at (1.5) should be [10]
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/css/css-transitions/animations/z-index-interpolation.html b/Tests/LibWeb/Text/input/wpt-import/css/css-transitions/animations/z-index-interpolation.html
new file mode 100644
index 00000000000..c7614536950
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/css/css-transitions/animations/z-index-interpolation.html
@@ -0,0 +1,130 @@
+<!DOCTYPE html>
+<meta charset="UTF-8">
+<title>z-index interpolation</title>
+<link rel="help" href="https://www.w3.org/TR/CSS2/visuren.html#z-index">
+<meta name="assert" content="z-index supports animation">
+
+<script src="../../../resources/testharness.js"></script>
+<script src="../../../resources/testharnessreport.js"></script>
+<script src="../../../css/support/interpolation-testcommon.js"></script>
+
+<style>
+body {
+  margin-top: 20px;
+}
+.layer-reference {
+  position: fixed;
+  top: 0px;
+  height: 100vh;
+  width: 50px;
+  background-color: rgba(255, 255, 255, 0.75);
+  font-family: sans-serif;
+  text-align: center;
+  padding-top: 5px;
+  border: 1px solid;
+}
+.parent {
+  z-index: 15;
+}
+.target {
+  position: relative;
+  width: 350px;
+  height: 10px;
+  z-index: -2;
+}
+.actual {
+  background-color: black;
+}
+.expected {
+  background-color: green;
+}
+</style>
+
+<body></body>
+
+<script>
+test_interpolation({
+  property: 'z-index',
+  from: neutralKeyframe,
+  to: '5',
+}, [
+  {at: -0.3, expect: '-4'},
+  {at: 0, expect: '-2'},
+  {at: 0.3, expect: '0'},
+  {at: 0.6, expect: '2'},
+  {at: 1, expect: '5'},
+  {at: 1.5, expect: '9'},
+]);
+
+test_no_interpolation({
+  property: 'z-index',
+  from: 'initial',
+  to: '5',
+});
+
+// We fail to inherit correctly due to style overadjustment: crbug.com/375982
+test_interpolation({
+  property: 'z-index',
+  from: 'inherit',
+  to: '5',
+}, [
+  {at: -0.3, expect: '18'},
+  {at: 0, expect: '15'},
+  {at: 0.3, expect: '12'},
+  {at: 0.6, expect: '9'},
+  {at: 1, expect: '5'},
+  {at: 1.5, expect: '0'},
+]);
+
+test_no_interpolation({
+  property: 'z-index',
+  from: 'unset',
+  to: '5',
+});
+
+test_interpolation({
+  property: 'z-index',
+  from: '-5',
+  to: '5'
+}, [
+  {at: -0.3, expect: '-8'},
+  {at: 0, expect: '-5'},
+  {at: 0.3, expect: '-2'},
+  {at: 0.6, expect: '1'},
+  {at: 1, expect: '5'},
+  {at: 1.5, expect: '10'},
+]);
+
+test_interpolation({
+  property: 'z-index',
+  from: '2',
+  to: '4'
+}, [
+  {at: -0.3, expect: '1'},
+  {at: 0, expect: '2'},
+  {at: 0.3, expect: '3'},
+  {at: 0.6, expect: '3'},
+  {at: 1, expect: '4'},
+  {at: 1.5, expect: '5'},
+]);
+
+test_interpolation({
+  property: 'z-index',
+  from: '-2',
+  to: '-4'
+}, [
+  {at: -0.3, expect: '-1'},
+  {at: 0, expect: '-2'},
+  {at: 0.1, expect: '-2'},
+  {at: 0.3, expect: '-3'},
+  {at: 0.6, expect: '-3'},
+  {at: 1, expect: '-4'},
+  {at: 1.5, expect: '-5'},
+]);
+
+test_no_interpolation({
+  property: 'z-index',
+  from: 'auto',
+  to: '10',
+});
+</script>