Show div when hover another div using only css - html

Question

I have searched the web for this one but didn't find anything similar.
Say we have div A and div B. When hover on div A, div b should be visible ON (should look like overwriting) div A and not placed under.
It should appear like only the content of div A has changed to content of div B.
How can this be done in pure CSS and HTML?
#container > div {
display: none
}
#container > div:first-child {
display: block
}
#container > div:hover + div {
display: block
<div id="container">
<div>A</div>
<div>B</div>
</div>

Answer 1

This will work, but only if the two divs are adjacent and b follows a.
#a:hover + #b {
background: #f00
}
<div id="a">Div A</div>
<div id="b">Div B</div>
If you have divs in between use ~
#a:hover ~ #b {
background: #f00
}
<div id="a">Div A</div>
<div id="c">Div C</div>
<div id="b">Div B</div>
To go the other way around, unfortunately you will need Javascript
// Pure Javascript
document.getElementById('b').onmouseover = function(){
document.getElementById('a').style.backgroundColor = '#f00';
}
document.getElementById('b').onmouseout = function(){
document.getElementById('a').style.backgroundColor = '';
}
// jQuery
$('#b').hover(
function(){$('#a').css('background', '#F00')},
function(){$('#a').css('background', '')}
);
Full fiddle http://jsfiddle.net/p7hLL/5/

Answer 2

If you don't want to use the selector + or ~ which aren't compatible with some browsers, it is just possible if the <div /> to show (e.g. div#b) is a child of the <div /> to hover (e.g. div#a).
<div id="a">
<div id="b"></div>
</div>
<style>
div#b {
display: none;
}
div#a:hover div#b {
display: block;
}
</style>

Answer 3

Its works, but your css should be like this
<style>
#b{display:none;}
div#a:hover div#b{display:inline}
</style>