diff --git a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
index d3a29d41f9..b2055922d9 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
+++ b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp
@@ -6,7 +6,6 @@
 
 #include <LibWeb/Bindings/CSSStyleSheetPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
-#include <LibWeb/CSS/CSSNamespaceRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/StyleComputer.h>
@@ -30,6 +29,12 @@ CSSStyleSheet::CSSStyleSheet(JS::Realm& realm, CSSRuleList& rules, MediaList& me
 
     for (auto& rule : *m_rules)
         rule->set_parent_style_sheet(this);
+
+    recalculate_namespaces();
+
+    m_rules->on_change = [this]() {
+        recalculate_namespaces();
+    };
 }
 
 JS::ThrowCompletionOr<void> CSSStyleSheet::initialize(JS::Realm& realm)
@@ -138,17 +143,40 @@ void CSSStyleSheet::set_style_sheet_list(Badge<StyleSheetList>, StyleSheetList*
     m_style_sheet_list = list;
 }
 
-Optional<StringView> CSSStyleSheet::namespace_filter() const
+Optional<StringView> CSSStyleSheet::default_namespace() const
 {
-    for (JS::NonnullGCPtr<CSSRule> rule : *m_rules) {
-        if (rule->type() == CSSRule::Type::Namespace) {
-            auto& namespace_rule = verify_cast<CSSNamespaceRule>(*rule);
-            if (!namespace_rule.namespace_uri().is_empty() && namespace_rule.prefix().is_empty())
-                return namespace_rule.namespace_uri().view();
-        }
-    }
+    if (m_default_namespace_rule)
+        return m_default_namespace_rule->namespace_uri().view();
 
     return {};
 }
 
+void CSSStyleSheet::recalculate_namespaces()
+{
+    for (JS::NonnullGCPtr<CSSRule> rule : *m_rules) {
+        // "Any @namespace rules must follow all @charset and @import rules and precede all other
+        // non-ignored at-rules and style rules in a style sheet.
+        // ...
+        // A syntactically invalid @namespace rule (whether malformed or misplaced) must be ignored."
+        // https://drafts.csswg.org/css-namespaces/#syntax
+        switch (rule->type()) {
+        case CSSRule::Type::Import:
+            continue;
+
+        case CSSRule::Type::Namespace:
+            break;
+
+        default:
+            // Any other types mean that further @namespace rules are invalid, so we can stop here.
+            return;
+        }
+
+        auto& namespace_rule = verify_cast<CSSNamespaceRule>(*rule);
+        if (!namespace_rule.namespace_uri().is_empty() && namespace_rule.prefix().is_empty())
+            m_default_namespace_rule = namespace_rule;
+
+        // FIXME: Store qualified namespace rules.
+    }
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h
index c6d031f71e..cffc45f0b8 100644
--- a/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h
+++ b/Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Function.h>
+#include <LibWeb/CSS/CSSNamespaceRule.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/CSS/CSSRuleList.h>
 #include <LibWeb/CSS/CSSStyleRule.h>
@@ -48,7 +49,7 @@ public:
 
     void set_style_sheet_list(Badge<StyleSheetList>, StyleSheetList*);
 
-    Optional<StringView> namespace_filter() const;
+    Optional<StringView> default_namespace() const;
 
 private:
     CSSStyleSheet(JS::Realm&, CSSRuleList&, MediaList&, Optional<AK::URL> location);
@@ -56,7 +57,10 @@ private:
     virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
+    void recalculate_namespaces();
+
     JS::GCPtr<CSSRuleList> m_rules;
+    JS::GCPtr<CSSNamespaceRule> m_default_namespace_rule;
 
     JS::GCPtr<StyleSheetList> m_style_sheet_list;
     JS::GCPtr<CSSRule> m_owner_css_rule;
diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
index e951d292f4..221c363a2d 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -225,7 +225,7 @@ Vector<MatchingRule> StyleComputer::filter_namespace_rules(DOM::Element const& e
     Vector<MatchingRule> filtered_rules;
 
     for (auto const& rule : rules) {
-        auto namespace_uri = rule.sheet->namespace_filter();
+        auto namespace_uri = rule.sheet->default_namespace();
         if (namespace_uri.has_value()) {
             if (namespace_uri.value() == element.namespace_uri())
                 filtered_rules.append(rule);