LibWeb: Absolutize CSS image URLs for computed style resolution

For getComputedStyle(), we must return an absolute URL for image style
values. We currently return the raw parsed URL.

This fixes loading the marker icons on https://usermap.serenityos.org.

Libraries/LibWeb/CSS/StyleValues/ImageStyleValue.cpp

@@ -7,6 +7,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibURL/Parser.h>
 #include <LibWeb/CSS/ComputedValues.h>
 #include <LibWeb/CSS/Fetch.h>
 #include <LibWeb/CSS/StyleValues/ImageStyleValue.h>
@@ -182,4 +183,33 @@ void ImageStyleValue::set_style_sheet(GC::Ptr<CSSStyleSheet> style_sheet)
     m_style_sheet = style_sheet;
 }
 
+ValueComparingNonnullRefPtr<CSSStyleValue const> ImageStyleValue::absolutized(CSSPixelRect const&, Length::FontMetrics const&, Length::FontMetrics const&) const
+{
+    if (m_url.url().is_empty())
+        return *this;
+
+    // FIXME: The spec has been updated to handle this better. The computation of the base URL here is roughly based on:
+    //        https://drafts.csswg.org/css-values-4/#style-resource-base-url
+    //        https://github.com/w3c/csswg-drafts/pull/12261
+    auto base_url = [&]() -> Optional<::URL::URL> {
+        if (m_style_sheet) {
+            return m_style_sheet->base_url()
+                .value_or_lazy_evaluated_optional([&]() { return m_style_sheet->location(); })
+                .value_or_lazy_evaluated_optional([&]() { return HTML::relevant_settings_object(*m_style_sheet).api_base_url(); });
+        }
+
+        if (m_document)
+            return m_document->base_url();
+
+        return {};
+    }();
+
+    if (base_url.has_value()) {
+        if (auto resolved_url = ::URL::Parser::basic_parse(m_url.url(), *base_url); resolved_url.has_value())
+            return ImageStyleValue::create(*resolved_url);
+    }
+
+    return *this;
+}
+
 }

Libraries/LibWeb/CSS/StyleValues/ImageStyleValue.h

@@ -53,6 +53,7 @@ private:
     ImageStyleValue(URL const&);
 
     virtual void set_style_sheet(GC::Ptr<CSSStyleSheet>) override;
+    virtual ValueComparingNonnullRefPtr<CSSStyleValue const> absolutized(CSSPixelRect const& viewport_rect, Length::FontMetrics const& font_metrics, Length::FontMetrics const& root_font_metrics) const override;
 
     void animate();
     Gfx::ImmutableBitmap const* bitmap(size_t frame_index, Gfx::IntSize = {}) const;

Tests/LibWeb/Text/expected/wpt-import/css/css-values/urls/resolve-relative-to-base.sub.txt

@@ -2,6 +2,7 @@ Harness status: OK
 
 Found 2 tests
 
-2 Fail
-Fail	base-relative URL: relative-image-url
+1 Pass
+1 Fail
+Pass	base-relative URL: relative-image-url
 Fail	base-relative URL: relative-image-variable-url

Tests/LibWeb/Text/expected/wpt-import/css/css-values/urls/resolve-relative-to-stylesheet.txt

@@ -2,7 +2,8 @@ Harness status: OK
 
 Found 3 tests
 
-3 Fail
-Fail	stylesheet-relative URL: stylesheet-relative-image
+1 Pass
+2 Fail
+Pass	stylesheet-relative URL: stylesheet-relative-image
 Fail	stylesheet-relative URL: stylesheet-relative-variable-image
 Fail	stylesheet-relative URL: stylesheet-relative-document-variable-image