diff --git a/lib/oga/css/parser.y b/lib/oga/css/parser.y
index 3d91e78..5bc66a1 100644
--- a/lib/oga/css/parser.y
+++ b/lib/oga/css/parser.y
@@ -117,16 +117,16 @@ rule
 
   pseudo_class
     # :root
-    : pseudo_name { s(:pseudo, val[0]) }
+    : pseudo_name { s(:pseudo, nil, val[0]) }
 
     # x:root
-    | path_member pseudo_name { s(:pseudo, val[1], val[0]) }
+    | path_member pseudo_name { s(:pseudo, val[0], val[1]) }
 
     # :nth-child(2)
-    | pseudo_name pseudo_args { s(:pseudo, val[0], nil, val[1]) }
+    | pseudo_name pseudo_args { s(:pseudo, nil, val[0], val[1]) }
 
     # x:nth-child(2)
-    | path_member pseudo_name pseudo_args { s(:pseudo, val[1], val[0], val[2]) }
+    | path_member pseudo_name pseudo_args { s(:pseudo, val[0], val[1], val[2]) }
     ;
 
   pseudo_name
diff --git a/spec/oga/css/parser/axes_spec.rb b/spec/oga/css/parser/axes_spec.rb
index 64a0572..ee595b1 100644
--- a/spec/oga/css/parser/axes_spec.rb
+++ b/spec/oga/css/parser/axes_spec.rb
@@ -77,7 +77,7 @@ describe Oga::CSS::Parser do
     example 'parse a pseudo class followed by the ~ axis' do
       parse_css('x:root ~ a').should == s(
         :following,
-        s(:pseudo, 'root', s(:test, nil, 'x')),
+        s(:pseudo, s(:test, nil, 'x'), 'root'),
         s(:test, nil, 'a')
       )
     end
@@ -86,7 +86,7 @@ describe Oga::CSS::Parser do
       parse_css('a ~ x:root').should == s(
         :following,
         s(:test, nil, 'a'),
-        s(:pseudo, 'root', s(:test, nil, 'x'))
+        s(:pseudo, s(:test, nil, 'x'), 'root')
       )
     end
   end
diff --git a/spec/oga/css/parser/classes_spec.rb b/spec/oga/css/parser/classes_spec.rb
index c879584..23fb13b 100644
--- a/spec/oga/css/parser/classes_spec.rb
+++ b/spec/oga/css/parser/classes_spec.rb
@@ -19,13 +19,13 @@ describe Oga::CSS::Parser do
     end
 
     example 'parse a selector using a class and a pseudo class' do
-      parse_css('.foo:root').should == s(:pseudo, 'root', s(:class, nil, 'foo'))
+      parse_css('.foo:root').should == s(:pseudo, s(:class, nil, 'foo'), 'root')
     end
 
     example 'parse a selector using a pseudo class and a class' do
       parse_css('x:root.foo').should == s(
         :class,
-        s(:pseudo, 'root', s(:test, nil, 'x')),
+        s(:pseudo, s(:test, nil, 'x'), 'root'),
         'foo'
       )
     end
diff --git a/spec/oga/css/parser/ids_spec.rb b/spec/oga/css/parser/ids_spec.rb
index d93ea6e..48cd85e 100644
--- a/spec/oga/css/parser/ids_spec.rb
+++ b/spec/oga/css/parser/ids_spec.rb
@@ -15,13 +15,13 @@ describe Oga::CSS::Parser do
     end
 
     example 'parse a selector using an ID and a pseudo class' do
-      parse_css('#foo:root').should == s(:pseudo, 'root', s(:id, nil, 'foo'))
+      parse_css('#foo:root').should == s(:pseudo, s(:id, nil, 'foo'), 'root')
     end
 
     example 'parse a selector using a pseudo class and an ID' do
       parse_css('x:root#foo').should == s(
         :id,
-        s(:pseudo, 'root', s(:test, nil, 'x')),
+        s(:pseudo, s(:test, nil, 'x'), 'root'),
         'foo'
       )
     end
diff --git a/spec/oga/css/parser/pseudo_classes_spec.rb b/spec/oga/css/parser/pseudo_classes_spec.rb
index b738993..81ab4e5 100644
--- a/spec/oga/css/parser/pseudo_classes_spec.rb
+++ b/spec/oga/css/parser/pseudo_classes_spec.rb
@@ -3,18 +3,18 @@ require 'spec_helper'
 describe Oga::CSS::Parser do
   context 'pseudo classes' do
     example 'parse the x:root pseudo class' do
-      parse_css('x:root').should == s(:pseudo, 'root', s(:test, nil, 'x'))
+      parse_css('x:root').should == s(:pseudo, s(:test, nil, 'x'), 'root')
     end
 
     example 'parse the :root pseudo class' do
-      parse_css(':root').should == s(:pseudo, 'root')
+      parse_css(':root').should == s(:pseudo, nil, 'root')
     end
 
     example 'parse the x:nth-child(1) pseudo class' do
       parse_css('x:nth-child(1)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:int, 1)
       )
     end
@@ -22,8 +22,8 @@ describe Oga::CSS::Parser do
     example 'parse the :nth-child(1) pseudo class' do
       parse_css(':nth-child(1)').should == s(
         :pseudo,
-        'nth-child',
         nil,
+        'nth-child',
         s(:int, 1)
       )
     end
@@ -31,8 +31,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(odd) pseudo class' do
       parse_css('x:nth-child(odd)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:odd)
       )
     end
@@ -40,8 +40,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(even) pseudo class' do
       parse_css('x:nth-child(even)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:even)
       )
     end
@@ -49,8 +49,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(n) pseudo class' do
       parse_css('x:nth-child(n)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:nth)
       )
     end
@@ -58,8 +58,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(-n) pseudo class' do
       parse_css('x:nth-child(-n)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:nth)
       )
     end
@@ -67,8 +67,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(2n) pseudo class' do
       parse_css('x:nth-child(2n)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:nth, s(:int, 2))
       )
     end
@@ -76,8 +76,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(2n+1) pseudo class' do
       parse_css('x:nth-child(2n+1)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:nth, s(:int, 2), s(:int, 1))
       )
     end
@@ -85,8 +85,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(2n-1) pseudo class' do
       parse_css('x:nth-child(2n-1)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:nth, s(:int, 2), s(:int, -1))
       )
     end
@@ -94,8 +94,8 @@ describe Oga::CSS::Parser do
     example 'parse the x:nth-child(-2n-1) pseudo class' do
       parse_css('x:nth-child(-2n-1)').should == s(
         :pseudo,
-        'nth-child',
         s(:test, nil, 'x'),
+        'nth-child',
         s(:nth, s(:int, -2), s(:int, -1))
       )
     end
@@ -103,16 +103,16 @@ describe Oga::CSS::Parser do
     example 'parse two pseudo selectors' do
       parse_css('x:focus:hover').should == s(
         :pseudo,
-        'hover',
-        s(:pseudo, 'focus', s(:test, nil, 'x'))
+        s(:pseudo, s(:test, nil, 'x'), 'focus'),
+        'hover'
       )
     end
 
     example 'parse a pseudo class with an identifier as the argument' do
       parse_css('x:lang(fr)').should == s(
         :pseudo,
-        'lang',
         s(:test, nil, 'x'),
+        'lang',
         s(:test, nil, 'fr')
       )
     end