figuring out the double colon notation

I have a video tag in my webpage, and I also add tracks (subtitles) to it.
I would like the subtitles to use a smaller font size.
So I asked on 'stackoverflow' and the suggestion I got was to use styles with a double colon in front of them. But when I put these into the css file of my project (I'm using Visual Studio Web Developer), they were rejected.
Visual Studio doesn't like the css that uses the double colon.
here are the styles that were provided:

::cue {
    color: yellow;
}
::cue(.small) {
    font-size: 80%;
}
.cue-display span.cue .small {
    font-size: 80%;
}
::cue(.customstyle) {
    color: red;
    font: bold;
}
.cue-display span.cue .customstyle {
    color: red;
    font: bold;
}