Transition on box-shadow on-off?

Question

http://lucasdebelder.be/googledoodle/ Here is a live version.

So as you can see on the portals there's a glow around it which is the box-shadow; I want to have it going on and off so it kinda feels more real, I've added transition: box-shadow ease-in-out; but it only does it at the start, after the page is loaded, and then just keeps glowing on.

Here's the relevant code. (portaal_links means portal left, and rechts means right, it's dutch)

HTML:

<div class="portaal portaal_links"></div>
<div class="portaal portaal_rechts"></div>

CSS:

/*portaal general*/
.portaal {
    position: absolute;
    width: 100px;
    height: 200px;
    border-radius: 50px / 100px;
    bottom: 315px;
}



/*portaal left*/
.portaal_links {
    background: radial-gradient(ellipse at center, #1e5799 0%,#2989d8 50%,#207cca 51%,#7db9e8 100%); /* W3C, IE10+, FF16+, Chrome26+, Opera12+, Safari7+ */
    transition: box-shadow ease-in-out;
    transition-delay: 1s;
    transition-duration: 1s;
    box-shadow: 0 0 55px #57B6FF;
    opacity: 0.75;
    left: 50px;
}


.portaal_rechts {
    background: radial-gradient(ellipse at center, rgba(243,197,189,1) 0%,rgba(232,108,87,1) 50%,rgba(234,40,3,1) 51%,rgba(255,102,0,1) 75%,rgba(199,34,0,1) 100%); /* W3C, IE10+, FF16+, Chrome26+, Opera12+, Safari7+ */
    box-shadow: 0 0 55px #FF6600;
    opacity: 0.55;
    left: 750px;
}

Answer 1

Use animation instead of transition.

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

Animation: https://www.w3schools.com/css/css3_animations.asp

.test {
  background: red;
  border-radius: 50%;
  width: 100px;
  height: 200px;
  animation: testing 3s linear infinite; }

@keyframes testing {
  25% { 
    box-shadow: 0 0 55px rgba(0,0,0,0.9); }
  75% {
    box-shadow: 0 0 0 transparent; }
}

<div class="test"></div>