What an amazing optical illusion you've stumbled upon!
It may seem like left: 50% is centering the elements, but in reality, it's shifting the left side of .black-box to the middle of .yellow-box. Adding transform: translate(-50%); to .black-box will truly center it within its parent.
.black-box {
width: 144px;
height: 84px;
position: absolute;
z-index: 2;
background: black;
left: 50%;
transform: translate(-50%);
}
.yellow-box {
height: 498px;
width: 33.33333333%;
position: relative;
background: yellow;
margin-bottom: 20px;
}
.cyan-box {
display: block;
position: absolute;
z-index: -1;
top: 0;
left: 0;
width: 654px;
height: 654px;
background: cyan;
}
.half {
width: 50%;
border-right: 1px black solid;
height: 100%;
}
<div class="yellow-box">
<div class="black-box">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>
The illusion dissipates when the page size changes. To see the true center of .yellow-box, I've added a line down the middle for reference.
Check out this example to compare the difference.
.yellow-box {
height: 100px;
width: 33.33333333%;
position: relative;
background: yellow;
margin-bottom: 20px;
}
.cyan-box {
display: block;
position: absolute;
z-index: -1;
top: 0;
left: 0;
width: 654px;
height: 100px;
background: cyan;
}
.black-box {
width: 144px;
height: 84px;
position: absolute;
z-index: 2;
background: black;
left: 50%;
}
.black-box-two {
width: 144px;
height: 84px;
position: absolute;
z-index: 2;
background: black;
left: 50%;
transform: translate(-50%);
}
.half {
width: 50%;
border-right: 1px black solid;
height: 100%;
}
* {
margin: 0;
padding: 0;
}
<div class="yellow-box">
<div class="black-box">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>
<div class="yellow-box">
<div class="black-box-two">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>
Remember that .black-box isn't actually aligning with its sibling as it appears to be doing.
If you prefer using margin: 0 auto, ensure .black-box has position: relative, as margins don't affect absolutely positioned elements.
.yellow-box {
height: 498px;
width: 33.33333333%;
position: relative;
background: yellow;
margin-bottom: 20px;
overflow: hidden;
}
.cyan-box {
display: block;
position: absolute;
z-index: -1;
top: 0;
left: 0;
width: 654px;
height: 654px;
background: cyan;
}
.black-box {
width: 144px;
height: 84px;
position: relative;
z-index: 2;
background: black;
margin: 0 auto;
}
.half {
width: 50%;
border-right: 1px black solid;
height: 100%;
}
<div class="yellow-box">
<div class="black-box">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>
By switching from absolute to relative positioning, margins come back into play. You can still use positioning properties like top, right, bottom, and left if needed.
Take a look at this example showcasing the two working solutions alongside your original code (left uses transform: translate(-50%), middle is the original, and right utilizes margin: 0 auto).
.yellow-box {
height: 100px;
width: 30%;
position: relative;
background: yellow;
margin-bottom: 20px;
overflow: hidden;
}
.cyan-box {
display: block;
position: absolute;
z-index: -1;
top: 0;
left: 0;
width: 654px;
height: 100px;
background: cyan;
}
.black-box {
width: 144px;
height: 84px;
position: relative;
z-index: 2;
background: black;
margin: 0 auto;
}
.black-box-two {
width: 144px;
height: 84px;
position: absolute;
z-index: 2;
background: black;
left: 50%;
margin: 0 auto;
}
.black-box-three {
width: 144px;
height: 84px;
position: absolute;
z-index: 2;
background: black;
left: 50%;
transform: translate(-50%);
}
.half {
width: 50%;
border-right: 1px black solid;
height: 100%;
}
* {
margin: 0;
padding: 0;
}
body {
display: flex;
justify-content: space-between;
}
<div class="yellow-box">
<div class="black-box">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>
<div class="yellow-box">
<div class="black-box-two">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>
<div class="yellow-box">
<div class="black-box-three">
</div>
<div class="cyan-box">
</div>
<div class="half"></div>
</div>