Can I use a :before or :after pseudo-element on an input field?

I am trying to use the :after CSS pseudo-element on an input field, but it does not work. If I use it with a span, it works OK.

<style type="text/css">
.mystyle:after {content:url(smiley.gif);}
.mystyle {color:red;}
</style>

This works (puts the smiley after "buu!" and before "some more")

<span class="mystyle">buuu!</span>a some more

This does not work - it only colors someValue in red, but there is no smiley.

<input class="mystyle" type="text" value="someValue">

What am I doing wrong? should I use another pseudo-selector?

Note: I cannot add a span around my input, because it is being generated by a third-party control.