CSS3 :after pseudo element with input

Just playing with :before and :after.

It seems I can't use them on/with an input button? Thought I could potentially use it do display an asterisk for required fields. Not necessarily what I will do, just an option

input {
    position: relative;
}
input:after {
    content: ''; 
    background: url(accept.png) 0 0 no-repeat; 
    height: 16px; 
    position: absolute; 
    right: -20px; 
    top: 5px; 
    width: 16px;
}

Same code works fine on an li

li {
    clear: both; 
    margin: 0 0 10px; 
    position: relative;
}
li:after {
    content: ''; 
    background: url(accept.png) 0 0 no-repeat; 
    height: 16px; 
    position: absolute; 
    right: -20px; 
    top: 5px; 
    width: 16px;   
}