LibWeb: Add CSSStyleSheet.{insert,delete,remove}Rule() APIs

Note that insertRule() is really just a big TODO right now.

Userland/Libraries/LibWeb/CSS/CSSRuleList.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <LibWeb/CSS/CSSRuleList.h>
+#include <LibWeb/DOM/ExceptionOr.h>
 
 namespace Web::CSS {
 
@@ -24,4 +25,28 @@ bool CSSRuleList::is_supported_property_index(u32 index) const
     return index < m_rules.size();
 }
 
+// https://drafts.csswg.org/cssom/#remove-a-css-rule
+DOM::ExceptionOr<void> CSSRuleList::remove_a_css_rule(u32 index)
+{
+    // 1. Set length to the number of items in list.
+    auto length = m_rules.size();
+
+    // 2. If index is greater than or equal to length, then throw an IndexSizeError exception.
+    if (index >= length)
+        return DOM::IndexSizeError::create("CSS rule index out of bounds.");
+
+    // 3. Set old rule to the indexth item in list.
+    auto& old_rule = m_rules[index];
+
+    // FIXME: 4. If old rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.
+    (void)old_rule;
+
+    // 5. Remove rule old rule from list at the zero-indexed position index.
+    m_rules.remove(index);
+
+    // FIXME: 6. Set old rule’s parent CSS rule and parent CSS style sheet to null.
+
+    return {};
+}
+
 }