style.css

/*
*  html5 doctor css reset | http://html5doctor.com/html-5-reset-stylesheet
*/
html,body,div,span,object,iframe,h1,h2,h3,h4,h5,h6,p,blockquote,pre,abbr,address,cite,code,del,dfn,em,img,ins,kbd,q,samp,small,strong,sub,sup,var,b,i,dl,dt,dd,ol,ul,li,fieldset,form,label,legend,table,caption,tbody,tfoot,thead,tr,th,td,article,aside,canvas,details,figcaption,figure,footer,header,hgroup,menu,nav,section,summary,time,mark,audio,video{margin:0;padding:0;border:0;outline:0;font-size:100%;vertical-align:baseline;background:transparent}
body{line-height:1}
article,aside,details,figcaption,figure,footer,header,hgroup,menu,nav,section{display:block}
nav ul{list-style:none}
blockquote,q{quotes:none}
blockquote:before,blockquote:after,q:before,q:after{content:none}
a{margin:0;padding:0;font-size:100%;vertical-align:baseline;background:transparent}
ins{background-color:#ff9;color:#000;text-decoration:none}
mark{background-color:#ff9;color:#000;font-style:italic;font-weight:bold}
del{text-decoration:line-through}
abbr[title],dfn[title]{border-bottom:1px dotted;cursor:help}
table{border-collapse:collapse;border-spacing:0}
hr{display:block;height:1px;border:0;border-top:1px solid #ccc;margin:1em 0;padding:0}
input,select{vertical-align:middle}

* {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

.board {
    display: flex;
    flex-wrap: wrap;
    height: 100vh;
    width: 100vh;
    margin: auto;
    border: 4px solid #000;
}

.cell {
    width: 25%;
    height: 33.333333333333336%;
    padding: 10px;
    background: yellow;
    perspective: 400px;
}

.card {
    position: relative;
    height: 100%;
    width: 100%;
    color: #fff;
    transform-style: preserve-3d;
    transition: all 0.8s;
}

.card.flipped {
    transform: rotateY(180deg);
}

.card div {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    backface-visibility: hidden;
}

.recto {
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 48px;
    background: red;
    transform: rotateY(180deg);
}

.verso {
    background: navy;
}