Center div inside div - html

Question

I know it's a noob questioin but I got stuck on this thing.
How to center vertically and horizontally "slide-loading" div in this situation?
.blueimp-gallery > .slides > .slide-loading
Element with class "blueimp-gallery" has position fixed.
Element with class "slides" has position relative.
Element with class "slide-loading" is a simple CSS preloader (without specified position).
.blueimp-gallery > .slides > .slide-loading {
border: 4px solid #f3f3f3;
border-radius: 50%;
border-top: 4px solid #3498db;
width: 60px;
height: 60px;
-webkit-animation: spin 1s linear infinite;
animation: spin 1s linear infinite;
}
#-webkit-keyframes spin {
0% { -webkit-transform: rotate(0deg); }
100% { -webkit-transform: rotate(360deg); }
}
#keyframes spin {
0% { transform: rotate(0deg); }
100% { transform: rotate(360deg); }
}

Answer 1

SOLUTION:
You can achieve this using the transform css property, using the following:
position: absolute;
top: calc(50% - 30px); //minus half the height of your spinner (30px)
left: calc(50% - 30px); //minus half the width of your spinner (30px)
transform: translate(-50%, -50%);
jsFIDDLE
CODE SNIPPET:
body {
margin: 0;
}
* {
box-sizing: border-box;
}
.rulerX,
.rulerY {
position: absolute;
z-index: 9999;
background-color: red;
}
.rulerX {
top: calc(50% - .5px);
height: 1px;
width: 100%;
}
.rulerY {
left: calc(50% - .5px);
width: 1px;
height: 100%;
}
.blueimp-gallery {
position: relative;
border: 3px solid royalblue;
height: 100vh;
box-shadow: 0 1px 3px rgba(0, 0, 0, 0.12), 0 1px 2px rgba(0, 0, 0, 0.24);
}
.slides {
height: 100%;
border: 3px dotted white;
position: relative;
background-color: #262626;
}
#-webkit-keyframes spin {
0% {
-webkit-transform: rotate(0deg);
}
100% {
-webkit-transform: rotate(360deg);
}
}
#keyframes spin {
0% {
transform: rotate(0deg);
}
100% {
transform: rotate(360deg);
}
}
.blueimp-gallery > .slides > .slide-loading {
border: 4px solid #f3f3f3;
border-radius: 50%;
border-top: 4px solid #3498db;
width: 60px;
height: 60px;
-webkit-animation: spin 1s linear infinite;
animation: spin 1s linear infinite;
position: absolute;
top: calc(50% - 30px);
left: calc(50% - 30px);
transform: translate(-50%, -50%);
}
<div class="blueimp-gallery">
<div class="rulerX"></div>
<div class="rulerY"></div>
<div class="slides">
<div class="slide-loading">
</div>
</div>
</div>