diff --git a/Libraries/LibWeb/CSS/CSSStyleSheet.cpp b/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
index 9e7168b6dc1..29ee3b1765d 100644
--- a/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
+++ b/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
@@ -11,6 +11,7 @@
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/CSSCounterStyleRule.h>
 #include <LibWeb/CSS/CSSImportRule.h>
+#include <LibWeb/CSS/CSSKeyframesRule.h>
 #include <LibWeb/CSS/CSSStyleRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
 #include <LibWeb/CSS/FontComputer.h>
@@ -165,7 +166,9 @@ WebIDL::ExceptionOr<unsigned> CSSStyleSheet::insert_rule(StringView rule, unsign
         // NOTE: The spec doesn't say where to set the parent style sheet, so we'll do it here.
         parsed_rule->set_parent_style_sheet(this);
 
-        if (!constructed() && owner_node() && owner_node()->is_html_style_element() && parsed_rule->type() == CSSRule::Type::Style)
+        if (!constructed() && owner_node() && owner_node()->is_html_style_element() && parsed_rule->type() == CSSRule::Type::Keyframes)
+            invalidate_owners_for_inserted_keyframes_rule(*this, as<CSSKeyframesRule>(*parsed_rule));
+        else if (!constructed() && owner_node() && owner_node()->is_html_style_element() && parsed_rule->type() == CSSRule::Type::Style)
             invalidate_owners_for_inserted_style_rule(*this, as<CSSStyleRule>(*parsed_rule), DOM::StyleInvalidationReason::StyleSheetInsertRule);
         else
             invalidate_owners(DOM::StyleInvalidationReason::StyleSheetInsertRule);
diff --git a/Libraries/LibWeb/CSS/StyleSheetInvalidation.cpp b/Libraries/LibWeb/CSS/StyleSheetInvalidation.cpp
index 38aa2e78c10..8e129600414 100644
--- a/Libraries/LibWeb/CSS/StyleSheetInvalidation.cpp
+++ b/Libraries/LibWeb/CSS/StyleSheetInvalidation.cpp
@@ -4,12 +4,17 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/CSS/CSSKeyframesRule.h>
 #include <LibWeb/CSS/CSSStyleRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
+#include <LibWeb/CSS/ComputedProperties.h>
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/SelectorEngine.h>
 #include <LibWeb/CSS/StyleScope.h>
 #include <LibWeb/CSS/StyleSheetInvalidation.h>
+#include <LibWeb/CSS/StyleValues/CustomIdentStyleValue.h>
+#include <LibWeb/CSS/StyleValues/StringStyleValue.h>
+#include <LibWeb/CSS/StyleValues/StyleValueList.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/ShadowRoot.h>
@@ -365,4 +370,175 @@ void invalidate_owners_for_inserted_style_rule(CSSStyleSheet const& style_sheet,
     }
 }
 
+static void for_each_tree_affected_by_shadow_root_stylesheet_change(
+    DOM::Node& root,
+    bool include_host,
+    bool include_light_dom_under_shadow_host,
+    Function<void(DOM::Node&)> const& callback)
+{
+    callback(root);
+
+    if (auto* shadow_root = as_if<DOM::ShadowRoot>(root)) {
+        auto* host = shadow_root->host();
+        if (!host)
+            return;
+
+        // Slotted selectors only reach the current host subtree, but :host combinators can also reach siblings rooted
+        // alongside the host. A bare :host selector, however, only needs the host itself.
+        if (include_host && include_light_dom_under_shadow_host)
+            callback(host->root());
+        else if (include_host || include_light_dom_under_shadow_host)
+            callback(*host);
+    }
+}
+
+static bool style_value_references_animation_name(StyleValue const& value, FlyString const& animation_name)
+{
+    if (value.is_custom_ident())
+        return value.as_custom_ident().custom_ident() == animation_name;
+    if (value.is_string())
+        return value.as_string().string_value() == animation_name;
+
+    if (!value.is_value_list())
+        return false;
+
+    for (auto const& item : value.as_value_list().values()) {
+        if (item->is_custom_ident() && item->as_custom_ident().custom_ident() == animation_name)
+            return true;
+        if (item->is_string() && item->as_string().string_value() == animation_name)
+            return true;
+    }
+
+    return false;
+}
+
+static bool element_or_pseudo_references_animation_name(DOM::Element const& element, FlyString const& animation_name)
+{
+    auto references_animation_name_in_properties = [&](CSS::ComputedProperties const& computed_properties) {
+        return style_value_references_animation_name(computed_properties.property(PropertyID::AnimationName), animation_name);
+    };
+
+    if (auto computed_properties = element.computed_properties(); computed_properties && references_animation_name_in_properties(*computed_properties))
+        return true;
+
+    for (u8 i = 0; i < to_underlying(CSS::PseudoElement::KnownPseudoElementCount); ++i) {
+        auto pseudo_element = static_cast<CSS::PseudoElement>(i);
+        if (auto computed_properties = element.computed_properties(pseudo_element); computed_properties && references_animation_name_in_properties(*computed_properties))
+            return true;
+    }
+
+    return false;
+}
+
+static void invalidate_elements_affected_by_inserted_keyframes_rule(DOM::Node& root, FlyString const& animation_name)
+{
+    auto invalidate_matching_element = [&](DOM::Element& element) {
+        // A new @keyframes rule only matters for elements or pseudo-elements that were already referencing the
+        // inserted animation-name.
+        if (element_or_pseudo_references_animation_name(element, animation_name))
+            element.set_needs_style_update(true);
+        return TraversalDecision::Continue;
+    };
+
+    if (root.is_document()) {
+        // Document styles are inherited by existing shadow trees too, so document-scoped @keyframes insertions must
+        // walk the shadow-including tree instead of stopping at shadow hosts.
+        root.for_each_shadow_including_inclusive_descendant([&](DOM::Node& node) {
+            if (auto* element = as_if<DOM::Element>(node))
+                return invalidate_matching_element(*element);
+            return TraversalDecision::Continue;
+        });
+        return;
+    }
+
+    root.for_each_in_inclusive_subtree_of_type<DOM::Element>(invalidate_matching_element);
+}
+
+static ShadowRootStylesheetEffects determine_shadow_root_stylesheet_effects_for_sheet(CSSStyleSheet const& style_sheet, DOM::ShadowRoot const& shadow_root)
+{
+    ShadowRootStylesheetEffects effects;
+
+    Vector<GC::Ptr<HTML::HTMLSlotElement const>> slots;
+    shadow_root.for_each_in_inclusive_subtree_of_type<HTML::HTMLSlotElement>([&](HTML::HTMLSlotElement const& slot) {
+        slots.append(slot);
+        return TraversalDecision::Continue;
+    });
+
+    style_sheet.for_each_effective_style_producing_rule([&](CSSRule const& rule) {
+        if (effects.may_match_shadow_host && effects.may_match_light_dom_under_shadow_host && effects.may_affect_assigned_nodes_via_slots)
+            return;
+
+        if (!is<CSSStyleRule>(rule))
+            return;
+
+        auto const& style_rule = as<CSSStyleRule>(rule);
+        for (auto const& selector : style_rule.absolutized_selectors()) {
+            effects.may_match_shadow_host |= selector->contains_pseudo_class(PseudoClass::Host);
+            effects.may_match_light_dom_under_shadow_host |= selector_may_match_light_dom_under_shadow_host(*selector);
+
+            if (!effects.may_affect_assigned_nodes_via_slots && !slots.is_empty()) {
+                for (auto const& slot : slots) {
+                    SelectorEngine::MatchContext context {
+                        .style_sheet_for_rule = style_sheet,
+                        .subject = *slot,
+                        .rule_shadow_root = &shadow_root,
+                    };
+                    if (SelectorEngine::matches(*selector, *slot, shadow_root.host(), context)) {
+                        effects.may_affect_assigned_nodes_via_slots = true;
+                        break;
+                    }
+                }
+            }
+
+            if (effects.may_match_shadow_host && effects.may_match_light_dom_under_shadow_host && effects.may_affect_assigned_nodes_via_slots)
+                return;
+        }
+    });
+
+    return effects;
+}
+
+ShadowRootStylesheetEffects determine_shadow_root_stylesheet_effects(DOM::ShadowRoot const& shadow_root)
+{
+    ShadowRootStylesheetEffects effects;
+
+    shadow_root.for_each_active_css_style_sheet([&](CSSStyleSheet& style_sheet) {
+        auto sheet_effects = determine_shadow_root_stylesheet_effects_for_sheet(style_sheet, shadow_root);
+        effects.may_match_shadow_host |= sheet_effects.may_match_shadow_host;
+        effects.may_match_light_dom_under_shadow_host |= sheet_effects.may_match_light_dom_under_shadow_host;
+        effects.may_affect_assigned_nodes_via_slots |= sheet_effects.may_affect_assigned_nodes_via_slots;
+    });
+
+    return effects;
+}
+
+void invalidate_owners_for_inserted_keyframes_rule(CSSStyleSheet const& style_sheet, CSSKeyframesRule const& keyframes_rule)
+{
+    for (auto& document_or_shadow_root : style_sheet.owning_documents_or_shadow_roots()) {
+        auto& style_scope = document_or_shadow_root->is_shadow_root()
+            ? as<DOM::ShadowRoot>(*document_or_shadow_root).style_scope()
+            : document_or_shadow_root->document().style_scope();
+        style_scope.invalidate_rule_cache();
+
+        if (!document_or_shadow_root->is_shadow_root() && document_or_shadow_root->entire_subtree_needs_style_update())
+            continue;
+
+        bool include_host = false;
+        bool include_light_dom_under_shadow_host = false;
+        if (auto* shadow_root = as_if<DOM::ShadowRoot>(*document_or_shadow_root)) {
+            auto effects = determine_shadow_root_stylesheet_effects(*shadow_root);
+            include_host = effects.may_match_shadow_host;
+            include_light_dom_under_shadow_host = effects.may_match_light_dom_under_shadow_host;
+        }
+
+        for_each_tree_affected_by_shadow_root_stylesheet_change(
+            *document_or_shadow_root,
+            include_host,
+            include_light_dom_under_shadow_host,
+            [&](DOM::Node& affected_root) {
+                invalidate_elements_affected_by_inserted_keyframes_rule(affected_root, keyframes_rule.name());
+            });
+    }
+}
+
 }
diff --git a/Libraries/LibWeb/CSS/StyleSheetInvalidation.h b/Libraries/LibWeb/CSS/StyleSheetInvalidation.h
index 28efa1beeee..4af5c5a0947 100644
--- a/Libraries/LibWeb/CSS/StyleSheetInvalidation.h
+++ b/Libraries/LibWeb/CSS/StyleSheetInvalidation.h
@@ -39,6 +39,12 @@ struct StyleSheetInvalidationSet {
     Vector<TrailingUniversalInvalidationRule> trailing_universal_rules;
 };
 
+struct ShadowRootStylesheetEffects {
+    bool may_match_shadow_host { false };
+    bool may_match_light_dom_under_shadow_host { false };
+    bool may_affect_assigned_nodes_via_slots { false };
+};
+
 // Extend `result` with the invalidation effects of `style_rule`'s selectors. Falls back to a whole-subtree
 // invalidation flag inside `result` when a selector is not amenable to targeted invalidation.
 void extend_style_sheet_invalidation_set_with_style_rule(StyleSheetInvalidationSet& result, CSSStyleRule const& style_rule);
@@ -54,8 +60,16 @@ WEB_API bool selector_may_match_light_dom_under_shadow_host(StringView selector_
 // as @property or @keyframes) whose effects are not captured by selector invalidation alone.
 void invalidate_root_for_style_sheet_change(DOM::Node& root, StyleSheetInvalidationSet const&, DOM::StyleInvalidationReason, bool force_broad_invalidation = false);
 
+// Summarize how any currently-active stylesheet in `shadow_root` can escape the shadow subtree. Used by mutation
+// paths that need host-side fallout derived from the whole shadow scope rather than a single sheet.
+ShadowRootStylesheetEffects determine_shadow_root_stylesheet_effects(DOM::ShadowRoot const&);
+
 // Apply a targeted invalidation to all documents and shadow roots that own `style_sheet` in response to inserting
 // `style_rule` into it.
 void invalidate_owners_for_inserted_style_rule(CSSStyleSheet const& style_sheet, CSSStyleRule const& style_rule, DOM::StyleInvalidationReason);
 
+// Apply a targeted invalidation to all documents and shadow roots that own `style_sheet` in response to inserting
+// `keyframes_rule` into it. Only elements already referencing the inserted animation-name are dirtied.
+void invalidate_owners_for_inserted_keyframes_rule(CSSStyleSheet const& style_sheet, CSSKeyframesRule const& keyframes_rule);
+
 }
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-host-invalidation-counters.txt b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-host-invalidation-counters.txt
new file mode 100644
index 00000000000..3d7ad311153
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-host-invalidation-counters.txt
@@ -0,0 +1 @@
+PASS: bare :host keyframes insertRule only invalidates the host (0 invalidations)
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-slotted-cross-sheet-invalidation.txt b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-slotted-cross-sheet-invalidation.txt
new file mode 100644
index 00000000000..2a792e72a2d
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-slotted-cross-sheet-invalidation.txt
@@ -0,0 +1,2 @@
+opacity before keyframes insertRule: 1
+opacity after keyframes insertRule: 0.25
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-tree-invalidation.txt b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-tree-invalidation.txt
new file mode 100644
index 00000000000..0f9a798c241
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-shadow-tree-invalidation.txt
@@ -0,0 +1,2 @@
+opacity before keyframes: 1
+opacity after keyframes: 0.25
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-starts-animation.txt b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-starts-animation.txt
new file mode 100644
index 00000000000..6e067f74da8
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-starts-animation.txt
@@ -0,0 +1,3 @@
+before: 1
+after: 1
+keyframes: 0
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-style-invalidation-counters.txt b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-style-invalidation-counters.txt
new file mode 100644
index 00000000000..00c0bac3063
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-keyframes-style-invalidation-counters.txt
@@ -0,0 +1 @@
+PASS: keyframes insertRule only invalidates animation users (1 invalidations)
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-shadow-root-slotted-invalidation.txt b/Tests/LibWeb/Text/expected/css/insert-rule-shadow-root-slotted-invalidation.txt
new file mode 100644
index 00000000000..ebaf2d1e785
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-shadow-root-slotted-invalidation.txt
@@ -0,0 +1,6 @@
+slotted before rule: rgb(0, 0, 0)
+slotted after rule: rgb(0, 0, 255)
+inherited before slot rule: rgb(0, 0, 0)
+inherited after slot rule: rgb(255, 0, 0)
+opacity before keyframes: 1
+opacity after keyframes: 0.25
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-host-invalidation-counters.html b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-host-invalidation-counters.html
new file mode 100644
index 00000000000..612fe3804d9
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-host-invalidation-counters.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<div id="host"></div>
+<script>
+    function settleAndReset(triggerElement) {
+        getComputedStyle(triggerElement).animationName;
+        getComputedStyle(triggerElement).animationName;
+        internals.resetStyleInvalidationCounters();
+    }
+
+    function addBystanders(parent, count) {
+        for (let i = 0; i < count; ++i) {
+            const bystander = document.createElement("div");
+            bystander.textContent = `bystander ${i}`;
+            parent.appendChild(bystander);
+        }
+    }
+
+    test(() => {
+        const host = document.getElementById("host");
+        const shadowRoot = host.attachShadow({ mode: "open" });
+        const sheet = new CSSStyleSheet();
+        sheet.replaceSync(`
+            :host {
+                animation-name: fade;
+                animation-duration: 1s;
+            }
+        `);
+        shadowRoot.adoptedStyleSheets = [sheet];
+
+        addBystanders(document.body, 25);
+        settleAndReset(host);
+
+        sheet.insertRule("@keyframes fade { from { opacity: 0.25; } to { opacity: 0.25; } }", sheet.cssRules.length);
+        getComputedStyle(host).animationName;
+        const invalidations = internals.getStyleInvalidationCounters().styleInvalidations;
+        if (invalidations <= 1)
+            println(`PASS: bare :host keyframes insertRule only invalidates the host (${invalidations} invalidations)`);
+        else
+            println(`FAIL: bare :host keyframes insertRule only invalidates the host (${invalidations} invalidations)`);
+    });
+</script>
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-slotted-cross-sheet-invalidation.html b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-slotted-cross-sheet-invalidation.html
new file mode 100644
index 00000000000..385b32419c2
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-slotted-cross-sheet-invalidation.html
@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<div id="host"><span id="slotted" class="item">slotted</span></div>
+<script>
+    asyncTest(async done => {
+        const host = document.getElementById("host");
+        const slotted = document.getElementById("slotted");
+        const shadowRoot = host.attachShadow({ mode: "open" });
+        const declarationsSheet = new CSSStyleSheet();
+        const keyframesSheet = new CSSStyleSheet();
+
+        shadowRoot.innerHTML = "<slot></slot>";
+        shadowRoot.adoptedStyleSheets = [declarationsSheet, keyframesSheet];
+
+        declarationsSheet.replaceSync(`
+            ::slotted(.item) {
+                animation-name: fade;
+                animation-duration: 1s;
+                animation-fill-mode: both;
+            }
+        `);
+
+        println(`opacity before keyframes insertRule: ${getComputedStyle(slotted).opacity}`);
+        keyframesSheet.insertRule("@keyframes fade { from { opacity: 0.25; } to { opacity: 0.25; } }", keyframesSheet.cssRules.length);
+        await animationFrame();
+        println(`opacity after keyframes insertRule: ${getComputedStyle(slotted).opacity}`);
+        done();
+    });
+</script>
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-tree-invalidation.html b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-tree-invalidation.html
new file mode 100644
index 00000000000..9df41b33cee
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-shadow-tree-invalidation.html
@@ -0,0 +1,31 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<style id="document-style"></style>
+<div id="host"></div>
+<script>
+    asyncTest(async done => {
+        const host = document.getElementById("host");
+        const shadowRoot = host.attachShadow({ mode: "open" });
+
+        shadowRoot.innerHTML = `
+            <style>
+                #target {
+                    animation-name: fade;
+                    animation-duration: 1s;
+                    animation-fill-mode: both;
+                }
+            </style>
+            <div id="target">target</div>
+        `;
+
+        const target = shadowRoot.getElementById("target");
+        const documentStyleSheet = document.getElementById("document-style").sheet;
+
+        println(`opacity before keyframes: ${getComputedStyle(target).opacity}`);
+        documentStyleSheet.insertRule("@keyframes fade { from { opacity: 0.25; } to { opacity: 0.25; } }", documentStyleSheet.cssRules.length);
+        await animationFrame();
+        println(`opacity after keyframes: ${getComputedStyle(target).opacity}`);
+
+        done();
+    });
+</script>
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-keyframes-starts-animation.html b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-starts-animation.html
new file mode 100644
index 00000000000..3b8904db721
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-starts-animation.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<style id="dynamic-style">
+    #target {
+        animation-duration: 1s;
+        animation-name: inserted;
+    }
+</style>
+<div id="target"></div>
+<script src="../include.js"></script>
+<script>
+    asyncTest(async done => {
+        const target = document.getElementById("target");
+        const sheet = document.getElementById("dynamic-style").sheet;
+
+        println(`before: ${target.getAnimations().length}`);
+
+        sheet.insertRule("@keyframes inserted { from { opacity: 0.25; } to { opacity: 0.75; } }", 1);
+        await animationFrame();
+
+        const animations = target.getAnimations();
+        println(`after: ${animations.length}`);
+        if (animations.length > 0)
+            println(`keyframes: ${animations[0].effect.getKeyframes().length}`);
+
+        done();
+    });
+</script>
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-keyframes-style-invalidation-counters.html b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-style-invalidation-counters.html
new file mode 100644
index 00000000000..2d3eae03209
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-keyframes-style-invalidation-counters.html
@@ -0,0 +1,43 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<style id="style-sheet">
+    #target {
+        animation-name: fade;
+        animation-duration: 1s;
+    }
+</style>
+<div id="target">target</div>
+<div>bystander 1</div>
+<div>bystander 2</div>
+<div>bystander 3</div>
+<script>
+    function settleAndReset(triggerElement) {
+        // Force style resolution before we observe counters so the inserted
+        // @keyframes rule is the only thing contributing to the measurement.
+        getComputedStyle(triggerElement).animationName;
+        getComputedStyle(triggerElement).animationName;
+        internals.resetStyleInvalidationCounters();
+    }
+
+    function addBystanders(parent, count) {
+        for (let i = 0; i < count; ++i) {
+            const bystander = document.createElement("div");
+            bystander.textContent = `bystander ${i + 4}`;
+            parent.appendChild(bystander);
+        }
+    }
+
+    test(() => {
+        const target = document.getElementById("target");
+        addBystanders(document.body, 25);
+        const sheet = document.getElementById("style-sheet").sheet;
+        settleAndReset(target);
+        sheet.insertRule("@keyframes fade { from { opacity: 0; } to { opacity: 1; } }", sheet.cssRules.length);
+        getComputedStyle(target).animationName;
+        const invalidations = internals.getStyleInvalidationCounters().styleInvalidations;
+        if (invalidations <= 1)
+            println(`PASS: keyframes insertRule only invalidates animation users (${invalidations} invalidations)`);
+        else
+            println(`FAIL: keyframes insertRule only invalidates animation users (${invalidations} invalidations)`);
+    });
+</script>
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-shadow-root-slotted-invalidation.html b/Tests/LibWeb/Text/input/css/insert-rule-shadow-root-slotted-invalidation.html
new file mode 100644
index 00000000000..35077266aca
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-shadow-root-slotted-invalidation.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<div id="host"><span id="slotted" class="item">slotted</span><span id="inherited">inherited</span></div>
+<script>
+    asyncTest(async done => {
+        const host = document.getElementById("host");
+        const slotted = document.getElementById("slotted");
+        const inherited = document.getElementById("inherited");
+        const shadowRoot = host.attachShadow({ mode: "open" });
+        shadowRoot.innerHTML = `
+            <style id="shadow-style">
+                ::slotted(.item) {
+                    animation-name: inserted;
+                    animation-duration: 1s;
+                    animation-fill-mode: both;
+                }
+            </style>
+            <slot></slot>
+        `;
+
+        const styleSheet = shadowRoot.getElementById("shadow-style").sheet;
+
+        println(`slotted before rule: ${getComputedStyle(slotted).color}`);
+        shadowRoot.appendChild(document.createElement("b"));
+        styleSheet.insertRule("::slotted(.item) { color: rgb(0, 0, 255); }", styleSheet.cssRules.length);
+        println(`slotted after rule: ${getComputedStyle(slotted).color}`);
+
+        println(`inherited before slot rule: ${getComputedStyle(inherited).color}`);
+        styleSheet.insertRule("slot { border-color: rgb(0, 0, 0); color: rgb(255, 0, 0); }", styleSheet.cssRules.length);
+        println(`inherited after slot rule: ${getComputedStyle(inherited).color}`);
+
+        println(`opacity before keyframes: ${getComputedStyle(slotted).opacity}`);
+        shadowRoot.appendChild(document.createElement("i"));
+        styleSheet.insertRule("@keyframes inserted { from { opacity: 0.25; } to { opacity: 0.25; } }", styleSheet.cssRules.length);
+        await animationFrame();
+        println(`opacity after keyframes: ${getComputedStyle(slotted).opacity}`);
+
+        done();
+    });
+</script>