diff --git a/lib/oga/css/parser.y b/lib/oga/css/parser.y
index c297dee..1fdd105 100644
--- a/lib/oga/css/parser.y
+++ b/lib/oga/css/parser.y
@@ -1,6 +1,13 @@
 ##
 # AST parser for CSS expressions.
 #
+# This parser does _not_ build a CSS specific AST, instead it directly produces
+# an XPath AST. This removes the need to transform the AST or generate
+# corresponding XPath expressions as a String.
+#
+# Similar to {Oga::XPath::Parser} this parser only takes String instances as
+# input.
+#
 class Oga::CSS::Parser
 
 token T_IDENT T_PIPE T_LBRACK T_RBRACK T_COLON T_SPACE T_LPAREN T_RPAREN T_MINUS
diff --git a/lib/oga/xpath/parser.y b/lib/oga/xpath/parser.y
index 0a5a9cc..a2f5542 100644
--- a/lib/oga/xpath/parser.y
+++ b/lib/oga/xpath/parser.y
@@ -2,6 +2,8 @@
 # AST parser for XPath expressions. The AST is built using {AST::Node}
 # instances.
 #
+# Unlike {Oga::XML::Parser} this parser only takes String instances as input.
+#
 class Oga::XPath::Parser
 
 token T_AXIS T_COLON T_COMMA T_FLOAT T_INT T_IDENT T_TYPE_TEST