Move divs in pairs on window resize - css

Question

This should be simple for you CSS gurus, but I really can't get this going. There are 4 boxes, example code:
<div id="wrapper">
<div id="firstPair">
<div style="width: 200px; float: left"></div>
<div style="width: 200px; float: left"></div>
</div>
<div id="secondPair">
<div style="width: 200px; float: left"></div>
<div style="width: 200px; float: left"></div>
</div>
</div>
When the window width is less than 800 only the rightmost div is moved, leaving them with 3 on top, and 1 on the next row.
I want the second two to go down the page as a pair. 2 on top, 2 on bottom, even if there is space for 3 next to eachother.

Answer 1

You need to set style for firstPair and secondPair elements
div[id$="Pair"] {
display: inline-block;
float: left;
}
<div id="wrapper">
<div id="firstPair">
<div style="width: 200px; float: left">s</div>
<div style="width: 200px; float: left">d</div>
</div>
<div id="secondPair">
<div style="width: 200px; float: left">f</div>
<div style="width: 200px; float: left">g</div>
</div>
</div>
And one more solution with shorten html, but some more use css
div[id$="Pair"] {
display: inline-block;
}
[id$="Pair"] > div {
width: 200px;
float: left;
background: lightgreen;
}
#wrapper {
text-align: center;
}
<div id="wrapper">
<div id="firstPair">
<div>s</div>
<div>d</div>
</div>
<div id="secondPair">
<div>f</div>
<div>g</div>
</div>
</div>
div[id$="Pair"] {
display: inline-block;
margin: 0;
}
[id$="Pair"] > div {
display: inline-block;
width: 200px;
margin: 2px 0;
background: lightgreen;
}
#wrapper {
text-align: center;
}
<div id="wrapper">
<div id="firstPair">
<div>1</div>
<div>2</div>
</div>
<div id="secondPair">
<div>3</div>
<div>4</div>
</div>
</div>

Answer 2

It is about BFC.
You might also float the containers :
#wrapper> div {
float:left;
}
<div id="wrapper">
<div id="firstPair">
<div style="width: 200px; float: left">1</div>
<div style="width: 200px; float: left">2</div>
</div>
<div id="secondPair">
<div style="width: 200px; float: left">3</div>
<div style="width: 200px; float: left">4</div>
</div>
</div>

Answer 3

Here's my solution. I tend to work in a fully responsive environment, so this will position them and be fully responsive on mobile. I also isolated the css, the inline colors are just for demo.
<style>
div#firstPair {
width: 100%;
max-width:400px;
float: left;
}
div#firstPair div{
width: 50%;
float: left;
}
div#secondPair {
width: 100%;
max-width: 400px;
float: left;
}
div#secondPair div{
width: 50%;
float: left;
}
</style>
<div id="wrapper">
<div id="firstPair">
<div style="background-color: blue;">first_1</div>
<div style="background-color: green;">first_2</div>
</div>
<div id="secondPair">
<div style="background-color: red;">second_1</div>
<div style="background-color: orange;">second_2</div>
</div>
<div style="clear: both;"></div>
</div>