I think as early as 6x via some hacks, but certainly from 7 on.

My search was:

"color:transparent" -background explorer

In Bing.... try it elsewhere, or refine it further.

Color:transparent only works from IE9 plus. It does not work in ie6, 7 or 8.

Gosh, I'm glad I only need to know this for an ebook. In case anyone wondered:

x {color: transparent; background: blahblah involving ditto marks;}

So

o wäre ich nur blahblah
<x>o wäre</x> <x>ich</x> <x>nur</x> otherblahblah
<x>o wäre</x> <x>ich</x> <x>nur</x> thirdblahblah

means that the "blahblah" elements line up exactly, regardless of font, but the beginning of each line only displays as ditto marks-- and if you copy-and-paste that second line by itself, the concealed text is included in the copy.

The difference between this and

x {visibility: hidden;}

is that "visibility: hidden" also suppresses display of the background. And "color: white" or similar conceals any bits of the background that happen to be covered with text, which creates an odd effect.

Fun fact which I've only recently discovered even though I've been making ebooks for years: In most browsers-- with the predictable exception of MSIE-- you can assign CSS properties to elements that do not exist in HTML. This is a great timesaver in the roughing-out stages because you don't have to decide right away which "real" element you want to assign the property to. Right now I've got <f> and <g>.