LibWeb: Add basic implementation of @page

This doesn't support selectors, and the only descriptors for now are for
margins.

Libraries/LibWeb/CSS/CSSPageRule.cpp

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/CSSPageRulePrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/CSSPageRule.h>
+#include <LibWeb/CSS/DescriptorID.h>
+#include <LibWeb/CSS/Serialize.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::CSS {
+
+GC_DEFINE_ALLOCATOR(CSSPageRule);
+
+GC::Ref<CSSPageRule> CSSPageRule::create(JS::Realm& realm, SelectorList&& selectors, GC::Ref<CSSPageDescriptors> style, CSSRuleList& rules)
+{
+    return realm.create<CSSPageRule>(realm, move(selectors), style, rules);
+}
+
+CSSPageRule::CSSPageRule(JS::Realm& realm, SelectorList&& selectors, GC::Ref<CSSPageDescriptors> style, CSSRuleList& rules)
+    : CSSGroupingRule(realm, rules, Type::Page)
+    , m_selectors(move(selectors))
+    , m_style(style)
+{
+    m_style->set_parent_rule(*this);
+}
+
+void CSSPageRule::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSPageRule);
+    Base::initialize(realm);
+}
+
+// https://drafts.csswg.org/cssom/#dom-csspagerule-selectortext
+String CSSPageRule::selector_text() const
+{
+    // The selectorText attribute, on getting, must return the result of serializing the associated selector list.
+    return serialize_a_group_of_selectors(m_selectors);
+}
+
+// https://drafts.csswg.org/cssom/#dom-csspagerule-selectortext
+void CSSPageRule::set_selector_text(StringView)
+{
+    // FIXME: On setting the selectorText attribute these steps must be run:
+    //  1. Run the parse a list of CSS page selectors algorithm on the given value.
+    //  2. If the algorithm returns a non-null value replace the associated selector list with the returned value.
+    //  3. Otherwise, if the algorithm returns a null value, do nothing.
+}
+
+// https://drafts.csswg.org/cssom/#ref-for-csspagerule
+String CSSPageRule::serialized() const
+{
+    auto& descriptors = *m_style;
+
+    StringBuilder builder;
+
+    // AD-HOC: There's no spec for this yet.
+    builder.append("@page "sv);
+    if (auto selector = selector_text(); !selector.is_empty())
+        builder.appendff("{} ", selector);
+    builder.append("{ "sv);
+    if (descriptors.length() > 0) {
+        builder.append(descriptors.serialized());
+        builder.append(' ');
+    }
+    builder.append("}"sv);
+
+    return builder.to_string_without_validation();
+}
+
+void CSSPageRule::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_style);
+}
+
+}