CSS: Create iOS style Pin Password box

Instead of images, have 4 input boxes. Fiddle here: http://jsfiddle.net/JLyn9/2/

<input type="password" maxlength=1 id="1" onkeyup="moveOnMax(this,'a')" />
<input type="password" maxlength=1 id="a" onkeyup="moveOnMax(this,'b')" />
<input type="password" maxlength=1 id="b" onkeyup="moveOnMax(this,'c')" />
<input type="password" maxlength=1 id="c" />

moveOnMax =function (field, nextFieldID) {
    if (field.value.length == 1) {
        document.getElementById(nextFieldID).focus();
    }
}

PS the JS function can be optimized.

EDIT/WARNING: This is not a solution to this problem. Please look at the other answers

Instead of using background-image for this purpose, use four input boxes with their type set to password, and than use jQuery to achieve some more user friendliness, if you don't need that functionality of auto focusing to next field you can simply omit the script out and achieve your style..

Demo

input[type=password] {
    padding: 10px;
    border: 1px solid #ddd;
    width: 50px;
    height: 50px;
    text-align: center;
    font-size: 30px;
}

jQuery (Optional for auto focus functionality)

$("input").keyup(function() {
    if($(this).val().length >= 1) {
      var input_flds = $(this).closest('form').find(':input');
      input_flds.eq(input_flds.index(this) + 1).focus();
    }
});

No need for jQuery, much cleaner IMO.

HTML

<form id="ios">
    <input type="password" maxlength="1" />
    <input type="password" maxlength="1"/>
    <input type="password" maxlength="1"/>
    <input type="password" maxlength="1"/>
</form>   

CSS

form#ios input[type=password]{
    height: 50px;
    width: 40px;
    text-align: center;
    font-size: 2em;
    border: 1px solid #000;
} 

Demo

http://jsfiddle.net/jerryhuangme/e9yhr/