<header>
  <h1>CSS <br/>Border animation ?</h1>
  <p>Hover me to find out !</p>
</header>
<span>=></span>
<header>
  <h1>CSS Border animation ?</h1>
  <p>Hover me to find out !</p>
</header>

body {
  text-align:center;
}
header {
  display:inline-table;
  vertical-align:middle;
  margin:1em ;
  padding:0.5em 1em;
  background:
    linear-gradient(
      to right,
      transparent 20%,
      turquoise 20%,
      turquoise 80%,
      transparent 80%
    ) bottom no-repeat,
    linear-gradient(
      to top,
      turquoise 0%,
      turquoise 300%
    ) right 300% no-repeat,
    linear-gradient(
      to right,
      turquoise 0%,
      turquoise 100%
    )   9000% 0 no-repeat,
    linear-gradient(
      to top,
      turquoise 0%,
      turquoise 300%
    ) left 9000% no-repeat
    ;  
  transition:background-size 2s ease-in;
}
header, header ~ header:hover {
  background-size: 
    100% 8px, 
    3px 0, 
    3px 3px, 
    3px 0;

}
header:hover, header ~ header {
  background-size: 
    200% 3px , 
    3px 100%, 
    100% 3px, 
    3px 100%;
  transition:background-size 2s ease-in-out;
}

span {
  font-size:4em;
  display:inline-block;
  vertical-align:middle;
  background:turquoise;
  border-radius:100%;
  box-shadow:0 0 5px;
}