在悬停时,您将看到此灰色线条

它在谷歌浏览器中很好,但这个灰色边框出现在其他浏览器中.
我怎么解决这个问题？

CSS

a,a:visited {color:#fff}

.btn {
  display: inline-block;
  -webkit-border-radius: 8px;
  -moz-border-radius: 8px;
  border-radius: 8px;
  -webkit-Box-shadow: 0 9px 0 #000000,0 13px 0 rgba(0,0.1);
  -moz-Box-shadow: 0 9px 0 #000000,0.1);
  Box-shadow: 0 9px 0 #000000,0.1);
  -webkit-transition: -webkit-Box-shadow .2s ease-in-out;
  -moz-transition: -moz-Box-shadow .2s ease-in-out;
  -o-transition: -o-Box-shadow .2s ease-in-out;
  transition: Box-shadow .2s ease-in-out;
  padding: 0px;
        background: black; /*  see ? */
}

.btn span {
  display: inline-block;
  padding: 22px 22px 11px;
  font-family: Arial,sans-serif;
  line-height: 1;
  text-shadow: 0 -1px 1px rgba(19,65,88,.8);
  background: #2e2e2e;
  -webkit-border-radius: 8px;
  -moz-border-radius: 8px;
  border-radius: 8px;
  -webkit-Box-shadow: inset 0 -1px 1px rgba(255,255,.15);
  -moz-Box-shadow: inset 0 -1px 1px rgba(255,.15);
  Box-shadow: inset 0 -1px 1px rgba(255,.15);
  -webkit-transition: -webkit-transform .2s ease-in-out;
  -moz-transition: -moz-transform .2s ease-in-out;
  -o-transition: -o-transform .2s ease-in-out;
  transition: transform .2s ease-in-out;
  color: #FFF;
        font-size: 32px;
        border: 0
}

.btn:hover {
  -webkit-Box-shadow: 0 8px 0 #000,0 12px 10px rgba(0,.3);
  -moz-Box-shadow: 0 8px 0 #000,.3);
  Box-shadow: 0 8px 0 #000,.3);
}

.btn:hover span {
  -webkit-transform: translate(0,-4px);
  -moz-transform: translate(0,-4px);
  -o-transform: translate(0,-4px);
  transform: translate(0,-4px);
}

.btn:active {
  -webkit-Box-shadow: 0 8px 0 #000,.3);
  -webkit-transition: -webkit-Box-shadow .2s ease-in-out;
  -moz-transition: -moz-Box-shadow .2s ease-in-out;
  -o-transition: -o-Box-shadow .2s ease-in-out;
  transition: Box-shadow .2s ease-in-out;
}
.btn:active span { 



  -webkit-transform: translate(0,0px);
  -moz-transform: translate(0,0px);
  -o-transform: translate(0,0px);
  transform: translate(0,0px);
}