LibWeb/CSS: Support converting CSSImageValue to a StyleValue

No known WPT improvements. CSSImageValue is a black box which we
implement the same as CSSStyleValue, so this may not be observably
different.

Libraries/LibWeb/CSS/CSSImageValue.cpp

@@ -7,6 +7,7 @@
 #include "CSSImageValue.h"
 #include <LibWeb/Bindings/CSSImageValuePrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/PropertyNameAndID.h>
 #include <LibWeb/CSS/StyleValues/StyleValue.h>
 #include <LibWeb/WebIDL/ExceptionOr.h>
 
@@ -38,4 +39,30 @@ WebIDL::ExceptionOr<String> CSSImageValue::to_string() const
     return Base::to_string();
 }
 
+// https://drafts.css-houdini.org/css-typed-om-1/#create-an-internal-representation
+WebIDL::ExceptionOr<NonnullRefPtr<StyleValue const>> CSSImageValue::create_an_internal_representation(PropertyNameAndID const& property) const
+{
+    // If value is a CSSStyleValue subclass,
+    //     If value does not match the grammar of a list-valued property iteration of property, throw a TypeError.
+    // NB: https://drafts.css-houdini.org/css-typed-om-1/#cssstylevalue-match-a-grammar doesn't list CSSImageValue, but
+    //     we should match <image>.
+    bool const matches_grammar = [&] {
+        if (property.is_custom_property()) {
+            // FIXME: If this is a registered custom property, check if that allows <image>.
+            return true;
+        }
+        return property_accepts_type(property.id(), ValueType::Image);
+    }();
+    if (!matches_grammar) {
+        return WebIDL::SimpleException { WebIDL::SimpleExceptionType::TypeError, MUST(String::formatted("Property '{}' does not accept <image>", property.name())) };
+    }
+
+    //     FIXME: If any component of property’s CSS grammar has a limited numeric range, and the corresponding part of value
+    //            is a CSSUnitValue that is outside of that range, replace that value with the result of wrapping it in a
+    //            fresh CSSMathSum whose values internal slot contains only that part of value.
+
+    //     Return the value.
+    return NonnullRefPtr { *source_value() };
+}
+
 }