Parsing support for the :first-child pseudo class.

lib/oga/css/parser.y

@@ -488,6 +488,19 @@ end
     return node
   end
 
+  ##
+  # Generates the AST for the `:first-child` selector.
+  #
+  # @return [AST::Node]
+  #
+  def on_pseudo_class_first_child
+    return s(
+      :eq,
+      s(:call, 'count', s(:axis, 'preceding-sibling', s(:test, nil, '*'))),
+      s(:int, 0)
+    )
+  end
+
   private
 
   ##

spec/oga/css/parser/pseudo_classes/first_child_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Oga::CSS::Parser do
+  context ':first-child pseudo class' do
+    example 'parse the :first-child pseudo class' do
+      parse_css(':first-child').should == parse_xpath(
+        'descendant-or-self::*[count(preceding-sibling::*) = 0]'
+      )
+    end
+  end
+end