CSS

position absolute center horizontally

.parent{
  position: relative;
}
.child{
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

position absolute horizontally center

/*if you want to align center on left attribute.
The same thing is for top alignment, you could use margin-top: (width/2 of your div), the concept is the same of left attribute.
It's important to set header element to position:relative.
try this:
*/
#logo {
    background:red;
    height:50px;
    position:absolute;
    width:50px;
    left:50%;
    margin-left:-25px;
}

/*If you would like to not use calculations you can do this:*/

#logo {
  background:red;
  width:50px;
  height:50px;
  position:absolute;
  left: 0;
  right: 0;
  margin: 0 auto;
}

absolute center css

.btn__play {
	position: absolute
    top: 50%
    left: 50%
    transform: translate(-50%, -50%)
}

center absolute element

.child {
    position: absolute;
    top: 50%;  /* position the top  edge of the element at the middle of the parent */
    left: 50%; /* position the left edge of the element at the middle of the parent */

    transform: translate(-50%, -50%); /* This is a shorthand of
                                         translateX(-50%) and translateY(-50%) */
}