Internet Explorer before version 8 understood the following CSS:

.transparent {      /* Internet Explorer < 9 */
    width: 100%;
    filter:alpha(opacity=50);
}

while Firefox had its own attribute:

.transparent {
    /* Firefox < 0.9 */
    -moz-opacity:0.5;
}

as did Safari:

.transparent {
    /* Safari < 2 */
    -khtml-opacity: 0.5;
}

Now, however, in CSS3, there’s a unified way to set an element’s transparency:

.transparent {
    /* IE >= 9, Firefox >= 0.9, Safari >= 2, Chrome, Opera >= 9 */
    opacity: 0.5;
}