LibWeb: Calculate font-size in NodeWidthStyle::apply_style()

Previously, this made the same "everything is px" assumption as in
`StyleProperties::load_font()`, so I've replaced it with the calculation
from there.

Userland/Libraries/LibWeb/Layout/Node.cpp

@@ -223,9 +223,13 @@ void NodeWithStyle::apply_style(const CSS::StyleProperties& specified_style)
  m_line_height = specified_style.line_height(*this);
 
  {
- // FIXME: This doesn't work right for relative font-sizes
- auto length = specified_style.length_or_fallback(CSS::PropertyID::FontSize, CSS::Length(10, CSS::Length::Type::Px));
- m_font_size = length.raw_value();
+ constexpr int default_font_size = 10;
+ auto parent_font_size = parent() == nullptr ? default_font_size : parent()->font_size();
+ auto length = specified_style.length_or_fallback(CSS::PropertyID::FontSize, CSS::Length(default_font_size, CSS::Length::Type::Px));
+ // FIXME: em sizes return 0 here, for some reason
+ m_font_size = length.resolved_or_zero(*this, parent_font_size).to_px(*this);
+ if (m_font_size == 0)
+ m_font_size = default_font_size;
  }
 
  auto bgimage = specified_style.property(CSS::PropertyID::BackgroundImage);