p,
img,
button,
h1,
h2,
h3,
h4,
h5,
h6,
header,
iframe,
a,
ul,
li,
.flex-sider {  
  opacity: 0 !important;
}

.visible {
    opacity: 1 !important; 
    transition: opacity 1.25s ease-in;
  }
  
  @keyframes opacity {
    from {
      opacity: 0
    }
  
    to {
      opacity: 1;
    }
  }