LibWeb: Improve fallback font selection

Try to find a font that has at least some of the requested properties.
This makes e.g. rfcs on tools.ietf.org easier to read since their
headers are now bold.

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -92,10 +92,15 @@ void StyleProperties::load_font() const
     auto family_parts = family_value.split(',');
     auto family = family_parts[0];
 
-    if (family.is_one_of("monospace", "ui-monospace"))
+    auto monospace = false;
+    auto bold = false;
+
+    if (family.is_one_of("monospace", "ui-monospace")) {
+        monospace = true;
         family = "Csilla";
-    else if (family.is_one_of("serif", "sans-serif", "cursive", "fantasy", "ui-serif", "ui-sans-serif", "ui-rounded"))
+    } else if (family.is_one_of("serif", "sans-serif", "cursive", "fantasy", "ui-serif", "ui-sans-serif", "ui-rounded")) {
         family = "Katica";
+    }
 
     int weight = 400;
     if (font_weight->is_identifier()) {
@@ -127,6 +132,8 @@ void StyleProperties::load_font() const
         weight = 900;
     }
 
+    bold = weight > 400;
+
     int size = 10;
     if (font_size->is_identifier()) {
         switch (static_cast<const IdentifierStyleValue&>(*font_size).id()) {
@@ -177,13 +184,27 @@ void StyleProperties::load_font() const
 
     if (!found_font) {
         dbgln("Font not found: '{}' {} {}", family, size, weight);
-        found_font = Gfx::FontDatabase::default_font();
+        found_font = font_fallback(monospace, bold);
     }
 
     m_font = found_font;
     FontCache::the().set(font_selector, *m_font);
 }
 
+RefPtr<Gfx::Font> StyleProperties::font_fallback(bool monospace, bool bold) const
+{
+    if (monospace && bold)
+        return Gfx::FontDatabase::default_bold_fixed_width_font();
+
+    if (monospace)
+        return Gfx::FontDatabase::default_fixed_width_font();
+
+    if (bold)
+        return Gfx::FontDatabase::default_bold_font();
+
+    return Gfx::FontDatabase::default_font();
+}
+
 float StyleProperties::line_height(const Layout::Node& layout_node) const
 {
     auto line_height_length = length_or_fallback(CSS::PropertyID::LineHeight, Length::make_auto());

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -76,6 +76,7 @@ private:
     Optional<CSS::Overflow> overflow(CSS::PropertyID) const;
 
     void load_font() const;
+    RefPtr<Gfx::Font> font_fallback(bool monospace, bool bold) const;
 
     mutable RefPtr<Gfx::Font> m_font;
 };