How do negative margins in CSS work and why is (margin-top:-5 != margin-bottom:5)?
Negative margins are valid in css and understanding their (compliant) behaviour is mainly based on the box model and margin collapsing. While certain scenarios are more complex, a lot of common mistakes can be avoided after studying the spec.
For instance, rendering of your sample code is guided by the css spec as described in calculating heights and margins for absolutely positioned non-replaced elements.
If I were to make a graphical representation, I'd probably go with something like this (not to scale):
The margin box lost 8px on the top, however this does not affect the content & padding boxes. Because your element is absolutely positioned, moving the element 8px up does not cause any further disturbance to the layout; with static in-flow content that's not always the case.
Bonus:
Still need convincing that reading specs is the way to go (as opposed to articles like this)? I see you're trying to vertically center the element, so why do you have to set margin-top:-8px; and not margin-top:-50%;?
Well, vertical centering in CSS is harder than it should be. When setting even top or bottom margins in %, the value is calculated as a percentage always relative to the width of the containing block. This is rather a common pitfall and the quirk is rarely described outside of w3 docos
I'll try to explain it visually:
/**
* explaining margins
*/
body {
padding: 3em 15%
}
.parent {
width: 50%;
width: 400px;
height: 400px;
position: relative;
background: lemonchiffon;
}
.parent:before,
.parent:after {
position: absolute;
content: "";
}
.parent:before {
top: 0;
bottom: 0;
left: 50%;
border-left: dashed 1px #ccc;
}
.parent:after {
left: 0;
right: 0;
top: 50%;
border-top: dashed 1px #ccc;
}
.child {
width: 200px;
height: 200px;
background: rgba(200, 198, 133, .5);
}
ul {
padding: 5% 20px;
}
.set1 .child {
margin: 0;
position: relative;
}
.set2 .child {
margin-left: 75px;
position: relative;
}
.set3 .child {
margin-left: -75px;
position: relative;
}
/* position absolute */
.set4 .child {
top: 50%;
left: 50%;
margin: 0;
position: absolute;
}
.set5 .child {
top: 50%;
left: 50%;
margin-left: 75px;
position: absolute;
}
.set6 .child {
top: 50%; /* level from which margin-top starts
- downwards, in the case of a positive margin
- upwards, in the case of a negative margin
*/
left: 50%; /* level from which margin-left starts
- towards right, in the case of a positive margin
- towards left, in the case of a negative margin
*/
margin: -75px;
position: absolute;
}<!-- content to be placed inside <body>…</body> -->
<h2><code>position: relative;</code></h2>
<h3>Set 1</h3>
<div class="parent set 1">
<div class="child">
<pre>
.set1 .child {
margin: 0;
position: relative;
}
</pre>
</div>
</div>
<h3>Set 2</h3>
<div class="parent set2">
<div class="child">
<pre>
.set2 .child {
margin-left: 75px;
position: relative;
}
</pre>
</div>
</div>
<h3>Set 3</h3>
<div class="parent set3">
<div class="child">
<pre>
.set3 .child {
margin-left: -75px;
position: relative;
}
</pre>
</div>
</div>
<h2><code>position: absolute;</code></h2>
<h3>Set 4</h3>
<div class="parent set4">
<div class="child">
<pre>
.set4 .child {
top: 50%;
left: 50%;
margin: 0;
position: absolute;
}
</pre>
</div>
</div>
<h3>Set 5</h3>
<div class="parent set5">
<div class="child">
<pre>
.set5 .child {
top: 50%;
left: 50%;
margin-left: 75px;
position: absolute;
}
</pre>
</div>
</div>
<h3>Set 6</h3>
<div class="parent set6">
<div class="child">
<pre>
.set6 .child {
top: 50%;
left: 50%;
margin: -75px;
position: absolute;
}
</pre>
</div>
</div>Margin is the spacing outside your element, just as padding is the spacing inside your element.
Setting the bottom margin indicates what distance you want below the current block. Setting a negative top margin indicates that you want negative spacing above your block. Negative spacing may in itself be a confusing concept, but just the way positive top margin pushes content down, a negative top margin pulls content up.