To apply opacity to all included tags, you can use a 1px PNG image with the desired color and 20% visibility as a background. If there is already an image present, adjust the alpha for the entire image. This approach should meet your requirements (after realizing it is important to examine the image rather than just the site design provided in the link).
Alternatively, you can place a div with opacity below other tags using CSS positioning and z-index. In this scenario, one div should contain all non-transparent content, while another adjacent div can have the desired opacity.