Name of this CSS syntax? : <p>Can someone please tell me what this syntax is called, I tried looking it up but didn't find it. Im refering to how the player class seems to be extended i.e. ".player:fullscreen". Thank you.</p>

<pre><code>.player {
    max-width: 750px; /n
    border: 5px solid rgba(0, 0, 0, 0.2);
    box-shadow: 0 0 20px rgba(0, 0, 0, 0.2);
    position: relative;
    font-size: 0;
    overflow: hidden;
}

/* This css is only applied when fullscreen is active */
.player:fullscreen {
    max-width: none;
    width: 100%;
}
.player:-webkit-full-screen {
    max-width: none;
    width: 100%;
}
</code></pre>
