#container:hover ~ #cube { background-color: yellow; }
#a:hover ~ #b {
background: #ccc
}
<div id="a">Div A</div>
<div>random other elements</div>
<div>random other elements</div>
<div>random other elements</div>
<div id="b">Div B</div>
#a:hover + #b {
background: #ccc
}
<div id="a">Div A</div>
<div id="b">Div B</div>