diff --git a/Tests/LibWeb/Ref/img-srcset-viewport-relative-sizes-ref.html b/Tests/LibWeb/Ref/img-srcset-viewport-relative-sizes-ref.html
new file mode 100644
index 0000000000..7c314bf765
--- /dev/null
+++ b/Tests/LibWeb/Ref/img-srcset-viewport-relative-sizes-ref.html
@@ -0,0 +1,6 @@
+
+
diff --git a/Tests/LibWeb/Ref/img-srcset-viewport-relative-sizes.html b/Tests/LibWeb/Ref/img-srcset-viewport-relative-sizes.html
new file mode 100644
index 0000000000..feba6ab258
--- /dev/null
+++ b/Tests/LibWeb/Ref/img-srcset-viewport-relative-sizes.html
@@ -0,0 +1,10 @@
+
+
diff --git a/Tests/LibWeb/Ref/manifest.json b/Tests/LibWeb/Ref/manifest.json
index 8c52f9f608..aad9eb7542 100644
--- a/Tests/LibWeb/Ref/manifest.json
+++ b/Tests/LibWeb/Ref/manifest.json
@@ -1,4 +1,5 @@
{
+ "img-srcset-viewport-relative-sizes.html": "img-srcset-viewport-relative-sizes-ref.html",
"square-flex.html": "square-ref.html",
"separate-borders-inline-table.html": "separate-borders-ref.html",
"opacity-stacking.html": "opacity-stacking-ref.html",
diff --git a/Userland/Libraries/LibWeb/HTML/SourceSet.cpp b/Userland/Libraries/LibWeb/HTML/SourceSet.cpp
index 5ccefead56..03c9de56ad 100644
--- a/Userland/Libraries/LibWeb/HTML/SourceSet.cpp
+++ b/Userland/Libraries/LibWeb/HTML/SourceSet.cpp
@@ -388,8 +388,15 @@ void SourceSet::normalize_source_densities(DOM::Element const& element)
{
// 1. Let source size be source set's source size.
auto source_size = [&] {
- if (!m_source_size.is_calculated())
+ if (!m_source_size.is_calculated()) {
+ // If the source size is viewport-relative, resolve it against the viewport right now.
+ if (m_source_size.value().is_viewport_relative()) {
+ return CSS::Length::make_px(m_source_size.value().viewport_relative_length_to_px(element.document().viewport_rect()));
+ }
+
+ // FIXME: Resolve font-relative lengths against the relevant font size.
return m_source_size.value();
+ }
// HACK: Flush any pending layouts here so we get an up-to-date length resolution context.
// FIXME: We should have a way to build a LengthResolutionContext for any DOM node without going through the layout tree.