Coin toss with JavaScript and HTML

That’s simply because you need to attach the event handler:

document.getElementById('click').onclick = click;

var heads = 0;
var tails = 0;
function click() {  
    x = (Math.floor(Math.random() * 2) == 0);
    if(x){
    	flip("heads");
    }else{
        flip("tails");
    }
};
function flip(coin) {
    document.getElementById("result").innerHTML = coin;
};

<button id="click" type="button">CLICK ME</button>
<p>
    You got: <span id="result"></span>
</p>