Use separate spec files for each pseudo class.

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

@@ -1,19 +1,7 @@
 require 'spec_helper'
 
 describe Oga::CSS::Parser do
-  context 'pseudo classes' do
+  context ':nth-child pseudo class' do
-    example 'parse the x:root pseudo class' do
-      parse_css('x:root').should == parse_xpath(
-        'descendant-or-self::x[not(parent::*)]'
-      )
-    end
-
-    example 'parse the :root pseudo class' do
-      parse_css(':root').should == parse_xpath(
-        'descendant-or-self::*[not(parent::*)]'
-      )
-    end
-
     example 'parse the x:nth-child(1) pseudo class' do
       parse_css('x:nth-child(1)').should == parse_xpath(
         'descendant-or-self::x[count(preceding-sibling::*) = 0]'

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

@@ -0,0 +1,17 @@
+require 'spec_helper'
+
+describe Oga::CSS::Parser do
+  context ':root pseudo class' do
+    example 'parse the x:root pseudo class' do
+      parse_css('x:root').should == parse_xpath(
+        'descendant-or-self::x[not(parent::*)]'
+      )
+    end
+
+    example 'parse the :root pseudo class' do
+      parse_css(':root').should == parse_xpath(
+        'descendant-or-self::*[not(parent::*)]'
+      )
+    end
+  end
+end