Using margin / padding to space span tag from the rest of the p tag

Overall just add display:block; to your span. You can leave your html unchanged.

Demo

You can do it with the following css:

p {
    font-size:24px; 
    font-weight: 300; 
    -webkit-font-smoothing: subpixel-antialiased; 
    margin-top:0px;
}

p span {
    font-size:16px; 
    font-style: italic; 
    margin-top:20px; 
    padding-left:10px; 
    display:inline-block;
}