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;
}