pulsing border

<style>
#scrollbox {
width: 300px;
height: 300px;
animation: borderPulse 5s infinite;
border: 2px solid;
overflow: auto;
}

@keyframes borderPulse {
25%: {border-color: #f00;}
50% {border-color: #0f0;}
75% {border-color: #00f;}
}
</style>
<div id="scrollbox">
text
</div>