CSS-Styling for nested lists

[Perangelot]

CSS-Styling for nested list does not work. For example,

1. One
  1. One.One
2. Two

does not create

1. One
  a. One.a
2. Two

if the external .css fille contains

  ol ol {
      list-style-type: lower-alpha;
  }

Adding

#+HTML_HEAD: <style type="text/css">
#+HTML_HEAD:  ol ol { list-style-type: lower-alpha; }
#+HTML_HEAD: </style>
1. One
  a. One.a
2. Two

does not work either. Both ways work for classical html export. Any ideas?