luke-else.co.uk/js/flip.js

var flipped = false;

function flipCard() {

    if (!flipped) {
        document.getElementById('card-front').style.transform = 'rotateX(-180deg)';
        document.getElementById('card-back').style.transform = 'rotateX(0deg)';
        flipped = true;
    }else{
        document.getElementById('card-front').style.transform = 'rotateX(0deg)';
        document.getElementById('card-back').style.transform = 'rotateX(-180deg)';
        flipped = false;
    }
}