Why is General sibling combinator not always working? - html

Question

I have four divs, and i want to change their width and height on hover so the one you are hovering over expands and all others shrink for how much hovered one expanded. I managed to get it working when i hover over first div, but when i try to do the same with other three nothing happens.
My HTML:
<div id="main">
<div id="mainOne">
<h3>text</h3>
</div>
<div id="mainTwo">
<h3>text2</h3>
</div>
<div id="mainThree">
<h3>text3</h3>
</div>
<div id="mainFour">
<h3>text4</h3>
</div>
</div>
My CSS:
/* HOVER 1 */
#mainOne:hover{
width:748px;
height:600px;
}
#mainOne:hover + #mainTwo{
width:248px;
height: 600px;
}
#mainOne:hover ~ #mainThree{
height:200px;
}
#mainOne:hover ~ #mainFour{
height:200px;
}
/* END HOVER 1 */
/* HOVER 2 */
#mainTwo:hover{
width:748px;
height:600px;
}
#mainTwo:hover + #mainOne{
width:248px;
height: 600px;
}
#mainTwo:hover + #mainThree{
height:200px;
}
#mainTwo:hover ~ #mainFour{
height:200px;
}
/* END HOVER 2 */
So when i hover over mainOne, everything changes, but when i hover over mainTwo just mainTwo changes and messes up everything else. What am i doing wrong?
Thanks.

Answer 1

CSS can only (currently) target elements that appear later in the DOM, therefore #mainTwo + #mainThree will work, but #mainTwo + #mainOne cannot.
To target previous siblings you'd have to wrap the siblings within another, parent, element and then style the previous siblings based on the hover of that parent.
div > div {
border: 1px solid #000;
padding: 0.5em 1em;
width: 80%;
margin: 0 auto;
color: #f00;
}
#main:hover > div {
width: 50%;
}
#main:hover > div:hover ~ div {
width: 50%;
}
#main:hover > div:hover {
width: 80%;
}
JS fiddle proof-of-concept

Answer 2

All your mainTwo rules with :hover use the adjacent sibling combinator.
Only #mainTwo + #mainThree can match as neither #mainOne or #mainFour are the next sibling to #mainTwo.
If you used the general sibling combinator (~) then you could match #mainFour too.
Since #mainOne precedes #mainTwo, you can't match it with a rule that depends on the existence of #mainTwo.