How can I use @keyframes for opacity ?

Answer 1

https://www.w3schools.com/cssref/css3_pr_animation-keyframes.asp

http://jjameson.mycpanel.princeton.edu/projects/2014/css-animations/presentation.html

Answer 2

Basic Declaration
@-webkit-keyframes NAME-YOUR-ANIMATION {
0% { opacity: 0; }
100% { opacity: 1; }
}
@-moz-keyframes NAME-YOUR-ANIMATION {
0% { opacity: 0; }
100% { opacity: 1; }
}
@-o-keyframes NAME-YOUR-ANIMATION {
0% { opacity: 0; }
100% { opacity: 1; }
}
@keyframes NAME-YOUR-ANIMATION {
0% { opacity: 0; }
100% { opacity: 1; }
}

Using:
#box {
-webkit-animation: NAME-YOUR-ANIMATION 5s infinite; /* Safari 4+ */
-moz-animation: NAME-YOUR-ANIMATION 5s infinite; /* Fx 5+ */
-o-animation: NAME-YOUR-ANIMATION 5s infinite; /* Opera 12+ */
animation: NAME-YOUR-ANIMATION 5s infinite; /* IE 10+, Fx 29+ */
}