Transform and Stacking Order - css

Question

I am trying to understand what is really happening “3d” world of CSS.
I made a simple example
Particularly the code which bugs me the most is:
.back {
background-color: tomato;
transform: rotateY(180deg);
z-index: 1;
}
The thing which is not clear to me is why when you hover over .inner, its background color (gold) is not visible?? If you remove the transform property from .back or if you set the rotateY to 0deg then the gold background color of the .inner is clearly visible.
Why is the transform property of .back changing the stacking order?
Logically it makes sense that children(.front and .back) should appear in front of their parent(.inner).
Also, I would like to know what really happens when you set transform-style to flat? Does that make parent and all of its children collapse into single “unit” where element with highest stacking order takes priority/visibility?

Answer 1

in your code :
.outer {
display: block;
width: 200px;
height: 200px;
border: 2px solid gold;
perspective: 1000px;
padding: 10px;
margin: 0 auto;
}
.inner {
position: relative;
height: 100%;
width: 100%;
transition: transform 2s linear;
transform-style: preserve-3d;
background-color: gold;
backface-visibility: visible;
transform: rotateY(50deg);
}
.sides {
position: absolute;
width: 100%;
height: 100%;
left: 0;
top: 0;
color: white;
backface-visibility: hidden;
}
.front {
background-color: blue;
transform: translateZ(20px)
}
.back {
background-color: tomato;
transform: rotateY(180deg) translateZ(10px);
}
.inner:hover {
transform: rotateY(180deg)
}
<div class="outer">
<div class="inner">
<div class="sides front">Front Side</div>
<div class="sides back">Back Side</div>
</div>
</div>
you are using
transform: rotateY(180deg) translateZ(10px);
The transforms are applied right to left, so first it goes to the front 10px. But after that, it rotates 180deg. (around the transform-origin that is constant). That makes the previous 10px go towards the back instead of to the front.
if the order is the inverse
transform: translateZ(10px) rotateY(180deg);
now the rotation is done first, and so the translation is unafected by it and goes to the front.
and No, sorry, z-index is not a substitute for 3-d transforms, if you want to use 3d transforms, translation is the only way to go ....
In your first example, z-index is useless, as can be seen easily
codepen with z-index removed
This works because you are setting
backface-visibility: hidden;
So only the face that is facing front will be visible