The opacity CSS property specifies the transparency of an element.
The opacity property can take a value from 0.0 - 1.0. The lower value, the more transparent:
Opacity is now a part of the CSS3 specifications, but it was present for a long time. However, older browsers have different ways of controlling the opacity or transparency.
Here is the most up to date syntax for CSS opacity in all current browsers.
The opacity property is often used together with the :hover selector to change the opacity on mouse-over:
Combining the both steps above you will get the opacity for all browsers.
When using the opacity property to add transparency to the background of an element, all of its child elements inherit the same transparency. This can make the text inside a fully transparent element hard to read: