Make text field update without page refresh?

I am trying to make a textfield update to a chosen font-family like this, $body_font is coming from a select box:

if(isset($body_font))
{
   $output .= "<div class='OSD-body-font-preview' style='font-family: $body_font'>";
   $output .= $typography_stored['preview'];
   $output .= "</div>";
}

It works but you have to save and refresh the page. Is there a way to have it update to the new font-family without those 2 steps?

Thank you,
-Eric