CSS for image inside visited link

Question

I'm trying to use a:visited img in a userstyle for YouTube, but it has no effect:

a:visited img {
    filter: grayscale(100%) !important;
}

On the other hand, if I instead use a:visited and a img, they work fine:

a:visited {
    color: red !important;
}

a img {
    filter: grayscale(100%) !important;
}

Why doesn't a:visited img work?