LibWeb: Handle CSS "ex" lengths (relative to font x-height)

These are pretty rare, but they do come up in some places and it's not
hard to support. The Gfx::Font information is approximate (and bad)
but we can fix that separately.

Libraries/LibWeb/CSS/Length.cpp

@@ -33,6 +33,8 @@ namespace Web::CSS {
 float Length::relative_length_to_px(const LayoutNode& layout_node) const
 {
     switch (m_type) {
+    case Type::Ex:
+        return m_value * layout_node.specified_style().font().x_height();
     case Type::Em:
         return m_value * layout_node.font_size();
     case Type::Rem:
@@ -49,6 +51,8 @@ const char* Length::unit_name() const
         return "px";
     case Type::Pt:
         return "pt";
+    case Type::Ex:
+        return "ex";
     case Type::Em:
         return "em";
     case Type::Rem:

Libraries/LibWeb/CSS/Length.h

@@ -39,6 +39,7 @@ public:
         Auto,
         Px,
         Pt,
+        Ex,
         Em,
         Rem,
     };
@@ -84,7 +85,7 @@ public:
     bool is_percentage() const { return m_type == Type::Percentage; }
     bool is_auto() const { return m_type == Type::Auto; }
     bool is_absolute() const { return m_type == Type::Px || m_type == Type::Pt; }
-    bool is_relative() const { return m_type == Type::Em || m_type == Type::Rem; }
+    bool is_relative() const { return m_type == Type::Ex || m_type == Type::Em || m_type == Type::Rem; }
 
     float raw_value() const { return m_value; }
     ALWAYS_INLINE float to_px(const LayoutNode& layout_node) const

Libraries/LibWeb/CSS/Parser/CSSParser.cpp

@@ -284,6 +284,9 @@ static CSS::Length parse_length(const CSS::ParsingContext& context, const String
     } else if (view.to_string().to_lowercase().ends_with("em")) {
         type = CSS::Length::Type::Em;
         value = try_parse_float(view.substring_view(0, view.length() - 2));
+    } else if (view.to_string().to_lowercase().ends_with("ex")) {
+        type = CSS::Length::Type::Ex;
+        value = try_parse_float(view.substring_view(0, view.length() - 2));
     } else if (view == "0") {
         type = CSS::Length::Type::Px;
         value = 0;