This question has been answered by isaac1. See the first response.
-webkit-clip-path: url("index.html#flag-right");
clip-path: url("index.html#flag-right");
<style>
.clip-flag-right {
-webkit-clip-path: url("#flag-right");
clip-path: url("#flag-right");
}
.clip-flag-left {
-webkit-clip-path: url("#flag-left");
clip-path: url("#flag-left");
}
</style>