Why do "negative margin and float applied elements" overlap? - html

Question

First of all, please look at this code.
I learned that this was a common way to realize liquid layout.
But I can not understand some of this code.
.container {
overflow: hidden;
}
main {
float: left;
width: 100%;
margin-right: -340px;
background: red;
}
.main-inner {
margin-right: 340px;
background: blue;
}
.sidebar {
float: right;
width: 340px;
background: green;
}
<div class="container">
<main>
<div class="main-inner">
<p class="main-title">Main</p>
</div>
</main>
<aside class="sidebar">
<div class="sidebar-inner">
sidebar
</div>
</aside>
</div>
Question 1
I understand that the negative margin has the effect of moving an element in the specified direction. However, when you run this code, the main element does not seem to be moving at all. Why is this?
Question 2
Since we set the width of the main element to 100%, I understand that the aside element hits the main element and that the main element and aside element can not be side by side.
So, I think that we prepare a horizontal width that can apply the aside element by applying negative margin, but the background color of the main element is applied in the same way as when the horizontal width is 100%. Why is the background color of the main element not (100% - aside width)? How is this series of rendering done?
Question 3
Which document on W3.org describes these actions? I tried looking, but I could not find any detailed information on them.
thank you.

Answer 1

Let's start by adding the properties one by one and see what is happening.
Intially we have this code with no margin applied and only float elements:
.container {
overflow: hidden;
background:yellow;
}
main {
float: left;
width: 100%;
background: red;
}
.main-inner {
background: blue;
}
.sidebar {
float: right;
width: 340px;
background: green;
}
<div class="container">
<main>
<div class="main-inner">
<p class="main-title">Main</p>
</div>
</main>
<aside class="sidebar">
<div class="sidebar-inner">
sidebar
</div>
</aside>
</div>
It's clear that you made the red element to be width:100% floating on the left and the green one to float on the right with a fixed width. You may also notice that p element is having a default margin that's why the blue is not totally covering the red.
Now if you add negative margin-right you will not move the element or decrease the width but you will pull the content from the right in order to overlap the element. Here is a basic illustration:
.box {
width: 200px;
height: 200px;
background: red;
float: left;
}
<div class="box" style="margin-right:-100px;height:220px">
</div>
<div class="box" style="background:blue;">
</div>
As you can see the blue box is overlapping the red one by exactly 100px because we applied -100px to the margin-right of the red box. Same logic will happen in your case, you applied a negative margin equal to the size of the sidebar so you created the need space to move the sidebar at the same level of the main element.
.container {
overflow: hidden;
background:yellow;
}
main {
float: left;
width: 100%;
background: red;
margin-right:-340px;
}
.main-inner {
background: blue;
}
.sidebar {
float: right;
width: 340px;
background: green;
}
<div class="container">
<main>
<div class="main-inner">
<p class="main-title">Main</p>
</div>
</main>
<aside class="sidebar">
<div class="sidebar-inner">
sidebar
</div>
</aside>
</div>
So the main element is still 100% width BUT the sidebar is overlapping it due to negative margin.
Now the last step is to add the margin inside the main and in this case it will reduce the width of the inner element to make the total (width + margin) always equal to the width of parent element (containing block)
.container {
overflow: hidden;
background:yellow;
}
main {
float: left;
width: 100%;
background: red;
margin-right:-340px;
}
.main-inner {
background: blue;
margin-right:340px;
}
.sidebar {
float: right;
width: 340px;
background: green;
}
<div class="container">
<main>
<div class="main-inner">
<p class="main-title">Main</p>
</div>
</main>
<aside class="sidebar">
<div class="sidebar-inner">
sidebar
</div>
</aside>
</div>
Here is another illustration of margin with block element non floated:
.container {
border: 2px solid;
max-width: 50vw;
margin: auto;
}
.first {
height: 100px;
background: red;
margin: 0 -50px;
}
.second {
height: 100px;
background: blue;
margin: 0 50px;
}
<div class="container">
<div class="first">
</div>
<div class="second">
</div>
</div>
In this case the width is increasing/decrasing due to margin because the logic is always: width + margin = width of containing block.
With elements like float and inline block the logic is the same but we won't have width changes because the width is defined either by the content or explicitly.
.container {
border: 2px solid;
display:inline-block;
}
.first {
float:left;
height: 100px;
background: red;
margin-right:-50px;
}
.second {
display:inline-block;
width:200px;
height: 120px;
background: blue;
margin-top:20px;
margin-right:-100px;
}
<div class="container">
<div class="first">
some text here
</div>
<div class="second">
</div>
</div>
Here the float element has a width defined by the content, the inline-block has a width equal to 200px. The negative margin is creating the overlap and the size of the parent element (the containing block) is equal to width + margins.
For the references:
8 Box model
9 Visual formatting model
10 Visual formatting model details
The above explanation is very simplifed. Refer to the specification links for a full and details explanation.

Answer 2

The odd placement from <main> comes from a browser css-rule
p {
display: block;
-webkit-margin-before: 1em;
-webkit-margin-after: 1em;
-webkit-margin-start: 0px;
-webkit-margin-end: 0px;
}
You can reset it using a css reset like normalize.css.
However, I recommend using display: flex. Some wonderful resources.
.container {
display: flex;
}
main {
width: 75%;
}
aside {
width: 25%;
}