[CSS21] text-decoration/visibility

it looks like text-decoration behaviour unspecified in this case:

<span style="color:blue;  text-decoration: underline;
">01
<span style="color:red;   text-decoration: underline; 
visibility:hidden;">11
<span style="color:green; text-decoration: none; 
visibility:visible;">21

1) should be area "11" decorated from "01"?
2) should be area "21" decorated from "01" or "11"?

FF/Opera treat this opposite way...