The osu! website is created in HTML5.
Proof: (this is firsts lines of the html page of this thread)
<!DOCTYPE html>
<html prefix="og: http://ogp.me/ns#">
<head>
<link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">
<link rel="icon" sizes="32x32" href="/favicon-32x32.png">
<link rel="icon" sizes="16x16" href="/favicon-16x16.png">
<link rel="manifest" href="/site.webmanifest">
<link rel="mask-icon" href="/safari-pinned-tab.svg" color="#e2609a">
<meta name="msapplication-TileColor" content="#603cba">
<meta name="theme-color" content="hsl(333, 10%, 40%)">
<meta charset="utf-8">
<meta name="description" content="osu! forum » Other » Off-Topic » ITT 2: We post shit that is neither funny nor interesting">
<meta name="keywords" content="osu, peppy, ouendan, elite, beat, agents, ds, windows, game, taiko, tatsujin, simulator, sim, xna, ddr, beatmania, osu!, osume">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
...