Не совсем уверен, что вы хотите. Просто пытаюсь угадать ...
.test {
font-size: 50px;
background-color: lightblue;
animation: rotate linear 6s infinite;
display: block;
width: 50px;
}
@keyframes rotate {
0% {transform: rotate(0deg)}
33% {transform: rotate(360deg)}
66% {transform: rotate(1080deg)}
100% {transform: rotate(7200deg)}
}
Another possible approach, making the speed increase smoother:
<div class="test">A</div>
.test {
font-size: 50px;
background-color: lightblue;
display: block;
width: 35px;
padding: 0px 14px;
animation: rotate 6s infinite;
animation-timing-function: cubic-bezier(.59,.07,.88,.64);
}
@keyframes rotate {
0% {transform: rotate(0deg)}
100% {transform: rotate(1800deg)}
}
<div class="test">A</div>