LibWeb/CSS: Add :unchecked pseudo-class

This just got added to the Selectors spec:

b78c97c19d

It's thus missing from the HTML spec and WPT, but I figured it was
simple enough to add.

Libraries/LibWeb/CSS/PseudoClasses.json

@@ -164,6 +164,9 @@
   "target": {
     "argument": ""
   },
+  "unchecked": {
+    "argument": ""
+  },
   "user-invalid": {
     "argument": ""
   },

Libraries/LibWeb/CSS/Selector.cpp

@@ -62,6 +62,7 @@ static bool can_selector_use_fast_matches(Selector const& selector)
                         PseudoClass::OnlyChild,
                         PseudoClass::Root,
                         PseudoClass::State,
+                        PseudoClass::Unchecked,
                         PseudoClass::Visited))
                     return false;
             } else if (!first_is_one_of(simple_selector.type,

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -616,6 +616,8 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         return element.matches_enabled_pseudo_class();
     case CSS::PseudoClass::Checked:
         return element.matches_checked_pseudo_class();
+    case CSS::PseudoClass::Unchecked:
+        return element.matches_unchecked_pseudo_class();
     case CSS::PseudoClass::Indeterminate:
         return matches_indeterminate_pseudo_class(element);
     case CSS::PseudoClass::HighValue:

Libraries/LibWeb/DOM/Element.cpp

@@ -1425,9 +1425,8 @@ bool Element::matches_checked_pseudo_class() const
     // The :checked pseudo-class must match any element falling into one of the following categories:
     // - input elements whose type attribute is in the Checkbox state and whose checkedness state is true
     // - input elements whose type attribute is in the Radio Button state and whose checkedness state is true
-    if (is<HTML::HTMLInputElement>(*this)) {
-        auto const& input_element = static_cast<HTML::HTMLInputElement const&>(*this);
-        switch (input_element.type_state()) {
+    if (auto* input_element = as_if<HTML::HTMLInputElement>(*this)) {
+        switch (input_element->type_state()) {
         case HTML::HTMLInputElement::TypeAttributeState::Checkbox:
         case HTML::HTMLInputElement::TypeAttributeState::RadioButton:
             return static_cast<HTML::HTMLInputElement const&>(*this).checked();
@@ -1437,8 +1436,27 @@ bool Element::matches_checked_pseudo_class() const
     }
 
     // - option elements whose selectedness is true
-    if (is<HTML::HTMLOptionElement>(*this)) {
-        return static_cast<HTML::HTMLOptionElement const&>(*this).selected();
+    if (auto* option_element = as_if<HTML::HTMLOptionElement>(*this)) {
+        return option_element->selected();
+    }
+    return false;
+}
+
+bool Element::matches_unchecked_pseudo_class() const
+{
+    // AD-HOC: There is no spec for this yet, so it's based on the spec for :checked, assuming that :unchecked applies to the same cases but with a `false` value.
+    if (auto* input_element = as_if<HTML::HTMLInputElement>(*this)) {
+        switch (input_element->type_state()) {
+        case HTML::HTMLInputElement::TypeAttributeState::Checkbox:
+        case HTML::HTMLInputElement::TypeAttributeState::RadioButton:
+            return !static_cast<HTML::HTMLInputElement const&>(*this).checked();
+        default:
+            return false;
+        }
+    }
+
+    if (auto* option_element = as_if<HTML::HTMLOptionElement>(*this)) {
+        return !option_element->selected();
     }
     return false;
 }

Libraries/LibWeb/DOM/Element.h

@@ -434,6 +434,7 @@ public:
     bool matches_enabled_pseudo_class() const;
     bool matches_disabled_pseudo_class() const;
     bool matches_checked_pseudo_class() const;
+    bool matches_unchecked_pseudo_class() const;
     bool matches_placeholder_shown_pseudo_class() const;
     bool matches_link_pseudo_class() const;
     bool matches_local_link_pseudo_class() const;

Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -167,7 +167,10 @@ void HTMLInputElement::set_checked(bool checked)
 
     invalidate_style(
         DOM::StyleInvalidationReason::HTMLInputElementSetChecked,
-        { { .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Checked } },
+        {
+            { .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Checked },
+            { .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Unchecked },
+        },
         {});
 
     if (auto* paintable = this->paintable())