html,
body {
    margin: 0px;
    padding: 0;
    background: #000;
    width: 100%;
    height: 100%;
    box-sizing: border-box;
    overflow: hidden;
}

main {
    width: 100vw; 
    height: 140vw; /* 1050/750 = 1.4 */
    max-height: 100vh;
    max-width: 71.4286vh; /* 750/1050 = 0.714286 */
    margin: auto;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    box-sizing: border-box;
}

#loader {
    width: 100%;
    height: 100%;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    z-index: 999;
    padding-top: 50%;
    background: #000;
    box-sizing: border-box;
}

#loader > img {
    display: none;
}

#loader > .white-logo {
    width: 80%;
    padding-bottom: 30.15%; 
    margin: 0 auto;
    margin-top: -30.1887%; /* 200/530 = 0.301887 */
    background-image: url(assets/loading-white.png);
    background-repeat: no-repeat;
    background-size: cover;
    box-sizing: border-box;
}

#loader > .color-logo {
    width: 80%;
    padding-bottom: 30.1887%; /* 200/530 = 0.301887 */
    margin: 0 auto;
    background-image: url(assets/loading-color.png);
    background-repeat: no-repeat;
    background-size: cover;
    box-sizing: border-box;
}

.container {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    width: 100%;
    height: 100%;
    -webkit-perspective: 1000px;
    -moz-perspective: 1000px;
    -o-perspective: 1000px;
    perspective: 1000px;
    transition: transform 0.5s;
    -webkit-transition: transform 0.5s linear;
    -moz-transition: transform 0.5s linear;
    -o-transition: transform 0.5s linear;
    transform: translateX(100vw);
    box-sizing: border-box;
}

.cover > nav {
    width: 60%;
    text-align: center;
    position: absolute;
    left: 0;
    right: 0;
    top: 58%;
    bottom: calc(10px + 12vmin);
    margin: 0 auto;
    display: flex;
    align-items: center;
    justify-content: space-around;
    flex-flow: column nowrap;
}

.cover > nav img {
    width: 100%;
    cursor: pointer;
}

.flashcard {
    width: 100vw; 
    height: 140vw; /* 1050/750 = 1.4 */
    max-height: 100vh;
    max-width: 71.4286vh; /* 750/1050 = 0.714286 */
    margin: auto;
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    transform-style: preserve-3d;
    -webkit-transform-style: preserve-3d;
    -moz-transform-style: preserve-3d;
    -o-transform-style: preserve-3d;
    transition: transform 0.3s;
    -webkit-transition: transform 0.3s linear;
    -moz-transition: transform 0.3s linear;
    -o-transition: transform 0.3s linear;
    box-sizing: border-box;
}

.flipped, .back {
    transform: rotateY(180deg);
    -webkit-transform: rotateY(180deg);
    -moz-transform: rotateY(180deg);
    -o-transform: rotateY(180deg);
    box-sizing: border-box;
}

.front, .back {
    position: absolute;
    width: 100%;
    height: 100%;
    text-align: center;
    -webkit-backface-visibility: hidden;
    -moz-backface-visibility: hidden;
    -o-backface-visibility: hidden;
    backface-visibility: hidden;
    display: block;
    background-size: cover;
    background-repeat: no-repeat;
    border: 5px solid #fff;
    box-sizing: border-box;
}

/*.front img, .back img {
    display: block;
    width: 100%;
    height: auto;
    border: 5px solid #fff;
    box-sizing: border-box;
}*/

main > nav {
    display: none;
}

.leftarrow {
    cursor: pointer;
    position: absolute;
    left: calc(5px + 2vmin);
    bottom: calc(10px + 2vmin);
    width: 10vmin;
    height: 10vmin;
    background-image: url(assets/BB_arrowleft_default.png);
    background-repeat: no-repeat;
    background-size: contain;
    box-sizing: border-box;
}

.rightarrow {
    cursor: pointer;
    position: absolute;
    right: calc(5px + 2vmin);
    bottom: calc(10px + 2vmin);
    width: 10vmin;
    height: 10vmin;
    background-image: url(assets/BB_arrowright_default.png);
    background-repeat: no-repeat;
    background-size: contain;
    box-sizing: border-box;
}


.container[data-page="cover"] .front {
    background-image: url(assets/Background.png);
}
.container[data-page="instruction"] .front {
    background-image: url(assets/BrainBuilders_InstruxFront.png);
}
.container[data-page="instruction"] .back {
    background-image: url(assets/BrainBuilders_InstruxBack.png);
}

/**** 0to1 ****/
.container[data-level="0to1"][data-page="1"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card1_front.png);
}
.container[data-level="0to1"][data-page="1"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card1_back.png);
}
.container[data-level="0to1"][data-page="2"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card2_front.png);
}
.container[data-level="0to1"][data-page="2"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card2_back.png);
}
.container[data-level="0to1"][data-page="3"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card3_front.png);
}
.container[data-level="0to1"][data-page="3"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card3_back.png);
}
.container[data-level="0to1"][data-page="4"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card4_front.png);
}
.container[data-level="0to1"][data-page="4"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card4_back.png);
}
.container[data-level="0to1"][data-page="5"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card5_front.png);
}
.container[data-level="0to1"][data-page="5"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card5_back.png);
}
.container[data-level="0to1"][data-page="6"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card6_front.png);
}
.container[data-level="0to1"][data-page="6"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card6_back.png);
}
.container[data-level="0to1"][data-page="7"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card7_front.png);
}
.container[data-level="0to1"][data-page="7"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card7_back.png);
}
.container[data-level="0to1"][data-page="8"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card8_front.png);
}
.container[data-level="0to1"][data-page="8"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card8_back.png);
}
.container[data-level="0to1"][data-page="9"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card9_front.png);
}
.container[data-level="0to1"][data-page="9"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card9_back.png);
}
.container[data-level="0to1"][data-page="10"] .front {
    background-image: url(assets/0to1/BrainBuilders_0to1card10_front.png);
}
.container[data-level="0to1"][data-page="10"] .back {
    background-image: url(assets/0to1/BrainBuilders_0to1card10_back.png);
}

/**** 2to3 ****/
.container[data-level="2to3"][data-page="1"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card1_front.png);
}
.container[data-level="2to3"][data-page="1"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card1_back.png);
}
.container[data-level="2to3"][data-page="2"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card2_front.png);
}
.container[data-level="2to3"][data-page="2"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card2_back.png);
}
.container[data-level="2to3"][data-page="3"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card3_front.png);
}
.container[data-level="2to3"][data-page="3"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card3_back.png);
}
.container[data-level="2to3"][data-page="4"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card4_front.png);
}
.container[data-level="2to3"][data-page="4"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card4_back.png);
}
.container[data-level="2to3"][data-page="5"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card5_front.png);
}
.container[data-level="2to3"][data-page="5"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card5_back.png);
}
.container[data-level="2to3"][data-page="6"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card6_front.png);
}
.container[data-level="2to3"][data-page="6"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card6_back.png);
}
.container[data-level="2to3"][data-page="7"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card7_front.png);
}
.container[data-level="2to3"][data-page="7"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card7_back.png);
}
.container[data-level="2to3"][data-page="8"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card8_front.png);
}
.container[data-level="2to3"][data-page="8"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card8_back.png);
}
.container[data-level="2to3"][data-page="9"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card9_front.png);
}
.container[data-level="2to3"][data-page="9"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card9_back.png);
}
.container[data-level="2to3"][data-page="10"] .front {
    background-image: url(assets/2to3/BrainBuilders_2to3card10_front.png);
}
.container[data-level="2to3"][data-page="10"] .back {
    background-image: url(assets/2to3/BrainBuilders_2to3card10_back.png);
}

/**** 4to5 ****/
.container[data-level="4to5"][data-page="1"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card1_front.png);
}
.container[data-level="4to5"][data-page="1"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card1_back.png);
}
.container[data-level="4to5"][data-page="2"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card2_front.png);
}
.container[data-level="4to5"][data-page="2"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card2_back.png);
}
.container[data-level="4to5"][data-page="3"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card3_front.png);
}
.container[data-level="4to5"][data-page="3"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card3_back.png);
}
.container[data-level="4to5"][data-page="4"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card4_front.png);
}
.container[data-level="4to5"][data-page="4"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card4_back.png);
}
.container[data-level="4to5"][data-page="5"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card5_front.png);
}
.container[data-level="4to5"][data-page="5"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card5_back.png);
}
.container[data-level="4to5"][data-page="6"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card6_front.png);
}
.container[data-level="4to5"][data-page="6"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card6_back.png);
}
.container[data-level="4to5"][data-page="7"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card7_front.png);
}
.container[data-level="4to5"][data-page="7"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card7_back.png);
}
.container[data-level="4to5"][data-page="8"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card8_front.png);
}
.container[data-level="4to5"][data-page="8"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card8_back.png);
}
.container[data-level="4to5"][data-page="9"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card9_front.png);
}
.container[data-level="4to5"][data-page="9"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card9_back.png);
}
.container[data-level="4to5"][data-page="10"] .front {
    background-image: url(assets/4to5/BrainBuilders_4to5card10_front.png);
}
.container[data-level="4to5"][data-page="10"] .back {
    background-image: url(assets/4to5/BrainBuilders_4to5card10_back.png);
}