Howard Rauscher responded to this IE 8 ticket that talks about how opacity and IE 8 strict mode do not jive:

Description — IE8 Strict Mode correctly omits the filter: alpha(opacity=xx) in CSS which allows the user to specify the opacity in pre-IE8 browsers but does not implement the CSS3 opacity setting. While I understand that opacity is part of the CSS3 spec which is not finalized, this leaves developers with an odd regression in functionality where it is no longer possible to change opacity on css elements (where as it was with IE 5.5, IE 6.0, IE 7.0, Mozilla Firefox, Apple Safari, among others).

Comments — So the fact that this has been labeled as by design suggests that IE8 will be the only browser produced in the last 10 or so years that will not support opacity in its strictest mode. Thats rediculous. I understand the wish to be standards compliant but how hard is it to implement reading the css3 opacity tag (even if it still makes use of the filter, at least it will exist as a future standards equivelant tag).

At some point standards has to give way to usability. Mozilla, Opera, Apple all realize that a few tags that maybe are not official CSS 2 spec still need to be available. If major functionality is missing from the standards compliant version of IE8, who will use it, even if it is standards compliant.

You’ll have a whole host of websites that are standards compliant but need a few features unavailable in standards compliant mode. So these websites will be setup to use IE7 mode. And then when IE9 comes out you’ll have to deal with compatibility issues all over again.

Internet Explorer 8, IE8, CSS, Opacity