CSSRule.type should not return values greater than 15

https://bugs.webkit.org/show_bug.cgi?id=249560
rdar://103531316

Reviewed by Sam Weinig.

"This enumeration is thus frozen in its current state, and no new new values will be added to
reflect additional at-rules; all at-rules beyond the ones listed above will return 0."

https://w3c.github.io/csswg-drafts/cssom/#the-cssrule-interface

* LayoutTests/imported/w3c/web-platform-tests/css/css-properties-values-api/at-property-cssom-expected.txt:
* Source/WebCore/css/CSSRule.cpp:
(WebCore::CSSRule::typeForCSSOM const):

Return 0 for unexposed types.
Rename for clarity.

* Source/WebCore/css/CSSRule.h:
(WebCore::CSSRule::type const): Deleted.
* Source/WebCore/css/CSSRule.idl:
* Source/WebCore/css/PropertySetCSSStyleDeclaration.cpp:
(WebCore::StyleRuleCSSStyleDeclaration::StyleRuleCSSStyleDeclaration):
* Source/WebCore/css/StyleRuleType.h:
* Source/WebKitLegacy/mac/DOM/DOMCSSRule.mm:
(-[DOMCSSRule type]):

Canonical link: https://commits.webkit.org/258128@main

LayoutTests/imported/w3c/web-platform-tests/css/css-properties-values-api/at-property-cssom-expected.txt

@@ -12,7 +12,7 @@ PASS Rule for --syntax-only has expected cssText
 PASS Rule for --inherits-only has expected cssText
 PASS Rule for --initial-value-only has expected cssText
 PASS Rule for --tab	tab has expected cssText
-FAIL CSSRule.type returns 0 assert_equals: expected 0 but got 21
+PASS CSSRule.type returns 0
 PASS Rule for --valid returns expected value for CSSPropertyRule.name
 PASS Rule for --valid-reverse returns expected value for CSSPropertyRule.name
 PASS Rule for --valid-universal returns expected value for CSSPropertyRule.name

Source/WebCore/css/CSSRule.cpp

@@ -36,6 +36,17 @@ struct SameSizeAsCSSRule : public RefCounted<SameSizeAsCSSRule> {
 
 static_assert(sizeof(CSSRule) == sizeof(SameSizeAsCSSRule), "CSSRule should stay small");
 
+unsigned short CSSRule::typeForCSSOM() const
+{
+    // "This enumeration is thus frozen in its current state, and no new new values will be
+    // added to reflect additional at-rules; all at-rules beyond the ones listed above will return 0."
+    // https://drafts.csswg.org/cssom/#the-cssrule-interface
+    if (styleRuleType() >= firstUnexposedStyleRuleType)
+        return 0;
+
+    return static_cast<unsigned short>(styleRuleType());
+}
+
 ExceptionOr<void> CSSRule::setCssText(const String&)
 {
     return { };

Source/WebCore/css/CSSRule.h

@@ -37,7 +37,7 @@ class CSSRule : public RefCounted<CSSRule> {
 public:
     virtual ~CSSRule() = default;
 
-    unsigned short type() const { return static_cast<unsigned short>(styleRuleType()); }
+    WEBCORE_EXPORT unsigned short typeForCSSOM() const;
 
     virtual StyleRuleType styleRuleType() const = 0;
     virtual String cssText() const = 0;

Source/WebCore/css/CSSRule.idl

@@ -31,7 +31,7 @@
     readonly attribute CSSRule? parentRule;
     readonly attribute CSSStyleSheet? parentStyleSheet;
 
-    readonly attribute unsigned short type;
+    [ImplementedAs=typeForCSSOM] readonly attribute unsigned short type;
 
     [ImplementedAs=Unknown] const unsigned short UNKNOWN_RULE = 0;
     [ImplementedAs=Style] const unsigned short STYLE_RULE = 1;

Source/WebCore/css/PropertySetCSSStyleDeclaration.cpp

@@ -397,7 +397,7 @@ Ref<MutableStyleProperties> PropertySetCSSStyleDeclaration::copyProperties() con
 StyleRuleCSSStyleDeclaration::StyleRuleCSSStyleDeclaration(MutableStyleProperties& propertySet, CSSRule& parentRule)
     : PropertySetCSSStyleDeclaration(propertySet)
     , m_refCount(1)
-    , m_parentRuleType(static_cast<StyleRuleType>(parentRule.type()))
+    , m_parentRuleType(parentRule.styleRuleType())
     , m_parentRule(&parentRule)
 {
     m_propertySet->ref();

Source/WebCore/css/StyleRuleType.h

@@ -44,13 +44,15 @@ enum class StyleRuleType : uint8_t {
     CounterStyle = 11,
     Supports = 12,
     FontFeatureValues = 14,
+    // Numbers above 15 are not exposed to the web.
     LayerBlock = 16,
-    LayerStatement = 17,
-    Container = 18,
-    FontPaletteValues = 19,
-    // Those at-rules (@swash, @annotation,...) don't have a specified number in the spec.
+    LayerStatement,
+    Container,
+    FontPaletteValues,
     FontFeatureValuesBlock,
     Property,
 };
 
+static constexpr auto firstUnexposedStyleRuleType = StyleRuleType::LayerBlock;
+
 } // namespace WebCore

Source/WebKit/WebProcess/InjectedBundle/API/gtk/DOM/WebKitDOMCSSRule.cpp

@@ -196,7 +196,7 @@ gushort webkit_dom_css_rule_get_rule_type(WebKitDOMCSSRule* self)
     WebCore::JSMainThreadNullState state;
     g_return_val_if_fail(WEBKIT_DOM_IS_CSS_RULE(self), 0);
     WebCore::CSSRule* item = WebKit::core(self);
-    gushort result = item->type();
+    gushort result = item->typeForCSSOM();
     return result;
 }
 

Source/WebKitLegacy/mac/DOM/DOMCSSRule.mm

@@ -56,7 +56,7 @@ - (void)dealloc
 - (unsigned short)type
 {
     WebCore::JSMainThreadNullState state;
-    return static_cast<unsigned short>(IMPL->type());
+    return IMPL->typeForCSSOM();
 }
 
 - (NSString *)cssText