This rounded corner DIV was created using CSS.

The image is in the background, so it has rounded corners. View the page source and click on the CSS to view the corner styles.

This is a transparent DIV. Choose your RGB color and set the alpha transparency level.
This one is set at:
background: rgba(255, 255, 255, 0.5);
The R = red, G = green and B = blue.
The A = alpha or alpha transparency, which is why you use a percentage.