LibWeb: Implement the :placeholder-shown pseudo-class

This matches if the element has a placeholder, and that placeholder is
currently visible. This applies to `<input>` and `<textarea>` elements,
but our `<textarea>` is very limited so does not support placeholders.

Tests/LibWeb/Ref/css-placeholder-shown-selector-ref.html

@@ -0,0 +1,9 @@
+<style>
+.placeholder-shown {
+    background-color: yellow;
+}
+</style>
+<input type="text" placeholder="hi" class="placeholder-shown">
+<input type="text" placeholder="hi" value="I like turtles">
+<input type="text">
+<input type="text" value="I like turtles">

Tests/LibWeb/Ref/css-placeholder-shown-selector.html

@@ -0,0 +1,9 @@
+<style>
+:placeholder-shown {
+    background-color: yellow;
+}
+</style>
+<input type="text" placeholder="hi">
+<input type="text" placeholder="hi" value="I like turtles">
+<input type="text">
+<input type="text" value="I like turtles">

Tests/LibWeb/Ref/manifest.json

@@ -9,6 +9,7 @@
     "css-gradient-currentcolor.html": "css-gradient-currentcolor-ref.html",
     "css-lang-selector.html": "css-lang-selector-ref.html",
     "css-local-link-selector.html": "css-local-link-selector-ref.html",
+    "css-placeholder-shown-selector.html": "css-placeholder-shown-selector-ref.html",
     "css-gradients.html": "css-gradients-ref.html",
     "css-read-only-read-write-selectors.html": "css-read-only-read-write-selectors-ref.html",
     "svg-symbol.html": "svg-symbol-ref.html",

Userland/Libraries/LibWeb/CSS/PseudoClasses.json

@@ -95,6 +95,9 @@
   "paused": {
     "argument": ""
   },
+  "placeholder-shown": {
+    "argument": ""
+  },
   "playing": {
     "argument": ""
   },

Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -7,6 +7,7 @@
 
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/SelectorEngine.h>
+#include <LibWeb/CSS/StyleProperties.h>
 #include <LibWeb/CSS/ValueID.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/Element.h>
@@ -497,6 +498,17 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         return !matches_read_write_pseudo_class(element);
     case CSS::PseudoClass::ReadWrite:
         return matches_read_write_pseudo_class(element);
+    case CSS::PseudoClass::PlaceholderShown: {
+        // https://html.spec.whatwg.org/multipage/semantics-other.html#selector-placeholder-shown
+        //  The :placeholder-shown pseudo-class must match any element falling into one of the following categories:
+        // - input elements that have a placeholder attribute whose value is currently being presented to the user.
+        if (is<HTML::HTMLInputElement>(element) && element.has_attribute(HTML::AttributeNames::placeholder)) {
+            auto const& input_element = static_cast<HTML::HTMLInputElement const&>(element);
+            return input_element.placeholder_element() && input_element.placeholder_value().has_value();
+        }
+        // - FIXME: textarea elements that have a placeholder attribute whose value is currently being presented to the user.
+        return false;
+    }
     }
 
     return false;