Your a:visited {}
definition must come before your general a {}
definition. You can use a:visited to set a color, but setting a text-decoration doesnt' work - but if you later set a general text-decoration for a elements, it does.
So:
a:visited {color: yellow;}
a {color:yellow; text-decoration: none; }
works (gives all links in yellow, no text decoration ever), but
a {color:yellow; text-decoration: none; }
a:visited {color: yellow;}
and
a {color:yellow; text-decoration: none; }
a:visited {color: yellow; text-decoration: none;}
don't (both give all links in yellow, but underlined)