Totally exploiting the fact invalid HTML is accepted by modern browsers. :P

chrismorgan · on June 30, 2015

Somewhat, but it can still be fitted in legal HTML 5 in 137 bytes:

    <!DOCTYPE html><title>X</title><script>s=0,t=Date.now()</script><p onclick="(e=Date.now()-t)<15e3?style.margin=++s+e%300+'px':alert(s)">X

(Note that the original version is at least 36 bytes larger than it need be: https://github.com/omnus/tiny-twitch/pull/1)