Hi Rémi,
Thank you very much for your kind words!
It is actually possible with a few very simple CSS rules. Thus custom CSS should do the trick for you:
[html]input.orig:focus::-webkit-input-placeholder { color:transparent !important; }
input.orig:focus:-moz-placeholder { color:transparent !important; }
input.orig:focus::-moz-placeholder { color:transparent !important; }
input.orig:focus:-ms-input-placeholder { color:transparent !important; }[/html]
I hope this helps 🙂