Dohpacity

Joel on Software has published the first draft of a short essay on usability, where he makes the salient point that:

Something is usable if it behaves exactly
as expected.

This principle of usability could apply to more than just UI design though. Take CSS syntax for example:

#my-box {
    -moz-opacity:0.6;
    filter:alpha(opacity=60);
}

So let's say we placed a 60% opacity on the my-box element. This opacity would apply to all the child elements of my-box too, including images. Maybe that's not so desirable then; so we need to add:

#my-box img {
    -moz-opacity:1;
    filter:alpha(opacity=100);
}

Nope. Doesn't work at all. You can't redefine an opacity on the child element of an element that already has opacity. Maybe there's a reason for this constraint, but that's irrelevant from a usability perspective. This syntax simply doesn't conform to expectations.