diff --git a/Libraries/LibWeb/CSS/CSSRuleList.cpp b/Libraries/LibWeb/CSS/CSSRuleList.cpp
index 1def5ad1731..76b9971f45e 100644
--- a/Libraries/LibWeb/CSS/CSSRuleList.cpp
+++ b/Libraries/LibWeb/CSS/CSSRuleList.cpp
@@ -90,7 +90,48 @@ WebIDL::ExceptionOr<unsigned> CSSRuleList::insert_a_css_rule(Variant<StringView,
     if (!new_rule)
         return WebIDL::SyntaxError::create(realm(), "Unable to parse CSS rule."_string);
 
-    // FIXME: 6. If new rule cannot be inserted into list at the zero-index position index due to constraints specified by CSS, then throw a HierarchyRequestError exception. [CSS21]
+    auto has_rule_of_type_other_than_specified_before_index = [&](Vector<CSSRule::Type> types, size_t index) {
+        for (size_t i = 0; i < index; i++) {
+            if (!any_of(types, [&](auto type) { return m_rules[i]->type() == type; }))
+                return true;
+        }
+        return false;
+    };
+
+    auto has_rule_of_type_at_or_after_index = [&](CSSRule::Type type, size_t index) {
+        for (size_t i = index; i < m_rules.size(); i++) {
+            if (m_rules[i]->type() == type)
+                return true;
+        }
+        return false;
+    };
+
+    // 6. If new rule cannot be inserted into list at the zero-indexed position index due to constraints specified by CSS, then throw a HierarchyRequestError exception. [CSS21]
+    // "Any @import rules must precede all other valid at-rules and style rules in a style sheet
+    // (ignoring @charset and @layer statement rules) and must not have any other valid at-rules
+    // or style rules between it and previous @import rules, or else the @import rule is invalid."
+    // https://drafts.csswg.org/css-cascade-5/#at-import
+    //
+    // "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.
+
+    auto rule_is_disallowed = false;
+    switch (new_rule->type()) {
+    case CSSRule::Type::LayerStatement:
+        break;
+    case CSSRule::Type::Import:
+        rule_is_disallowed = has_rule_of_type_other_than_specified_before_index({ CSSRule::Type::Import, CSSRule::Type::LayerStatement }, index);
+        break;
+    case CSSRule::Type::Namespace:
+        rule_is_disallowed = has_rule_of_type_at_or_after_index(CSSRule::Type::Import, index) || has_rule_of_type_other_than_specified_before_index({ CSSRule::Type::Import, CSSRule::Type::Namespace, CSSRule::Type::LayerStatement }, index);
+        break;
+    default:
+        rule_is_disallowed = has_rule_of_type_at_or_after_index(CSSRule::Type::Import, index) || has_rule_of_type_at_or_after_index(CSSRule::Type::Namespace, index);
+        break;
+    }
+
+    if (rule_is_disallowed)
+        return WebIDL::HierarchyRequestError::create(realm(), "Cannot insert rule at specified index."_string);
 
     // 7. If new rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.
     if (new_rule->type() == CSSRule::Type::Namespace && any_of(m_rules, [](auto existing_rule) { return existing_rule->type() != CSSRule::Type::Import && existing_rule->type() != CSSRule::Type::Namespace; }))
diff --git a/Tests/LibWeb/Text/expected/css/insert-rule-constraints.txt b/Tests/LibWeb/Text/expected/css/insert-rule-constraints.txt
new file mode 100644
index 00000000000..4a0707bb020
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/css/insert-rule-constraints.txt
@@ -0,0 +1,5 @@
+Inserting import rules after style rule throws: HierarchyRequestError
+Inserting namespace rules after style rule throws: HierarchyRequestError
+Inserting namespace rules before import rule throws: HierarchyRequestError
+Inserting style rules before import rule throws: HierarchyRequestError
+Inserting style rules before namespace rule throws: HierarchyRequestError
diff --git a/Tests/LibWeb/Text/input/css/insert-rule-constraints.html b/Tests/LibWeb/Text/input/css/insert-rule-constraints.html
new file mode 100644
index 00000000000..0e6ea22f4d5
--- /dev/null
+++ b/Tests/LibWeb/Text/input/css/insert-rule-constraints.html
@@ -0,0 +1,78 @@
+<!DOCTYPE html>
+<html>
+    <style>
+        div {
+        }
+    </style>
+    <style>
+        div {
+        }
+    </style>
+    <style>
+        @import url("foo.css");
+    </style>
+    <style>
+        @namespace abc url("https://www.w3.org/1999/xhtml");
+    </style>
+    <style>
+        @media (all) {
+            * {
+            }
+        }
+    </style>
+    <script src="../include.js"></script>
+    <script>
+        test(() => {
+            try {
+                document.styleSheets[0].insertRule('@import url("foo.css");', 1);
+                println(
+                    "Fail! insertRule should not allow inserting import rules after style rules."
+                );
+            } catch (e) {
+                println(`Inserting import rules after style rule throws: ${e.name}`);
+            }
+
+            try {
+                document.styleSheets[1].insertRule(
+                    '@namespace abc url("https://www.w3.org/1999/xhtml");',
+                    1
+                );
+                println(
+                    "Fail! insertRule should not allow inserting namespace rules after style rules."
+                );
+            } catch (e) {
+                println(`Inserting namespace rules after style rule throws: ${e.name}`);
+            }
+
+            try {
+                document.styleSheets[2].insertRule(
+                    '@namespace abc url("https://www.w3.org/1999/xhtml");',
+                    0
+                );
+                println(
+                    "Fail! insertRule should not allow inserting namespace rules before import rule."
+                );
+            } catch (e) {
+                println(`Inserting namespace rules before import rule throws: ${e.name}`);
+            }
+
+            try {
+                document.styleSheets[2].insertRule("div { color: red; }", 0);
+                println(
+                    "Fail! insertRule should not allow inserting style rules before import rule."
+                );
+            } catch (e) {
+                println(`Inserting style rules before import rule throws: ${e.name}`);
+            }
+
+            try {
+                document.styleSheets[3].insertRule("div { color: red }", 0);
+                println(
+                    "Fail! insertRule should not allow inserting style rules before namespace rule."
+                );
+            } catch (e) {
+                println(`Inserting style rules before namespace rule throws: ${e.name}`);
+            }
+        });
+    </script>
+</html>