I've found out that the kellum method doesn't really work with html button elements. More precisely, it works but need more text indent. To recap, this is the techniqhe:
text-indent: 100%;
white-space: nowrap;
overflow: hidden;
With button elems, I've got to text-indent more, say 200%, it depends, it seems not to have a specific rule, but surely more than 100%. Why?