Why did this previously valid CSS suddenly become an unknown directive?

Question

I had the following CSS in an old web site:

@-o-keyframes rainbowGlow {
from {
         color: whitesmoke;
     }
13% {
        color: red;
    }
25% {
        color: orange;
    }
38% {
        color: yellow;
    }
50% {
        color: green;
    }
63% {
        color: blue;
    }
75% {
        color: indigo;
    }
88% {
        color: violet;
    }
to {
       color: white;
   }
}

Trying to use it in a new website, though, using the same CSS, I now get, "Validation (CSS 3.0): "-o-keyframes" is not a known '@' directive."

Why? How to rectify?