LibWeb: Make the initial containing block style have `display:block`

We always create a Layout::InitialContainingBlock for the ICB, but in a
future where we always honor the CSS::Display everywhere, we need to
make sure everyone has the right display values.

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1212,6 +1212,7 @@ NonnullRefPtr<StyleProperties> StyleComputer::create_document_style() const
     absolutize_values(style, nullptr, {});
     style->set_property(CSS::PropertyID::Width, CSS::LengthStyleValue::create(CSS::Length::make_px(viewport_rect().width())));
     style->set_property(CSS::PropertyID::Height, CSS::LengthStyleValue::create(CSS::Length::make_px(viewport_rect().height())));
+    style->set_property(CSS::PropertyID::Display, CSS::IdentifierStyleValue::create(CSS::ValueID::Block));
     return style;
 }