html {
  margin: 0;
  padding: 0;
  width: 100%;
}

body {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
}

#conteneur {
  height: 100%;
  display: flex;
  align-items: center;
  justify-content: center;
}

#main {
  position: relative;
  z-index: 0;
}

.bouton1 {
  position: absolute;
  margin-top: -50%;
  margin-left: -10%;
  z-index: 100;
}

.bouton2 {
  position: absolute;
  margin-top: -47%;
  margin-left: -10%;
  z-index: 100;
}

#main img { display: none }
#main img.first { display: block }


@media screen and (max-width: 720px) {
img {
  max-width: 100%; 
  }

#main {
  margin-top: 50px;
  }

.bouton1 {
  position: absolute;
  margin-top: 5%;
  margin-left: 30%;
  z-index: 100;
  }

.bouton2 {
  position: absolute;
  margin-top: 5%;
  margin-left: 50%;
  z-index: 100;
  }
}