Adding a border to an IFrame is no biggie - you do it like this e.g.:
border: 4px solid #000; -moz-border-radius: 15px; border-radius: 15px; >
border: 4px solid #000; -moz-border-radius: 15px; border-radius: 15px;
You miss overflow and position properties. This should work:
overflow
position
border: 4px solid #000; -moz-border-radius: 15px; border-radius: 15px; overflow: hidden; position: relative;