Thanks buddy. Well, using the first one,
<a href="#" onclick="document.getElementById('printletter<?=$i?>').submit();" class="box">Imprimer</a>
i'm getting in IE, Object doesn't support this property or method. I've removed the name attribute from form.
FF tells me document.getElementById("printletter0") has no properties
And yes, that's why I avoid to use doctype strict. Transitional is more flexible.