Center CSS element across platforms

Site URL: http://gregorylassale.com

Hello, 

I am using a blend of HMTL, CSS and JS to add scrolldown arrows on certain pages of my site. Here is the CSS:

.scroll a {
  position: absolute;
  bottom: 20px;
  left: 46.5%;
  z-index: 2;
  display: inline-block;
  -webkit-transform: translate(0, -50%);
  transform: translate(0, -50%);
  color: black;
  font : normal 400 20px/1 'montserrat', sans-serif;
  letter-spacing: .1em;
  text-decoration: none;
  transition: opacity .3s;
}
.scroll a:hover {
  opacity: .5;
}

#arrow a {
  padding-top: 70px;
}
#arrow a span {
  position: absolute;
  top: 0;
  left: 50%;
  width: 24px;
  height: 24px;
  margin-left: -12px;
  border-left: 1px solid black;
  border-bottom: 1px solid black;
  -webkit-transform: rotate(-45deg);
  transform: rotate(-45deg);
  -webkit-animation: sdb05 1.5s infinite;
  animation: sdb05 1.5s infinite;
  box-sizing: border-box;
}
@-webkit-keyframes sdb05 {
  0% {
    -webkit-transform: rotate(-45deg) translate(0, 0);
    opacity: 0;
  }
  50% {
    opacity: 1;
  }
  100% {
    -webkit-transform: rotate(-45deg) translate(-20px, 20px);
    opacity: 0;
  }
}
@keyframes sdb05 {
  0% {
    transform: rotate(-45deg) translate(0, 0);
    opacity: 0;
  }
  50% {
    opacity: 1;
  }
  100% {
    transform: rotate(-45deg) translate(-20px, 20px);
    opacity: 0;
  }
}

 I played with the left position a bit and it looks center on desktop, but it's a bit off to the right on mobile. Is there CSS property that allows to center elements across all platforms? I'd like it center relative to the rest of the content on the page.

Thank you

There's also text in a HTML code block I need to center.