The Opacity Four-Pack

Before one opacity property worked everywhere, every faded box carried the real opacity line plus four legacy fallbacks and hoped one of them stuck.

In 2026: Partly. Exactly one line does the work now: opacity. The Mozilla, KHTML and Internet Explorer alpha() versions all parse to nothing, so the box below is forty percent opaque by the strength of a single modern property standing where four used to crowd. The IE filter line is why so many old pages had jagged text on faded elements.

Where it came from: The cross-browser opacity incantation of roughly 2003. Only the unprefixed property, standardised in CSS Color 3, still functions. MDN ↗

<style>
.ghost {
  opacity: 0.4;                                 /* the standard, and the survivor */
  -moz-opacity: 0.4;                            /* Netscape 6 / early Firefox      */
  -khtml-opacity: 0.4;                          /* Safari's Konqueror-era engine   */
  filter: alpha(opacity=40);                    /* Internet Explorer 4 to 8        */
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=40)"; /* IE8 */
  background: #cc0000; color: #fff; padding: 20px;
}
</style>

<div class="ghost">Forty percent opaque. One line of five is doing it.</div>