This (http://jsfiddle.net/77RRA/1/) is working, while this (http://jsfiddle.net/77RRA/) is not.
Isn't clearfix supposed to substitute the line <div style="clear: both;"></div>
?
Isn't clearfix supposed to substitute the line
<div style="clear: both;"></div>
Yes. The clearfix is there to avoid a non-semantic empty tag. However, for this to work you need to place it on the parent element. (Example)
In your case however, it does not address the problem that siblings will ignore the floated element. This is not the intend of clearfix, you simply add clear:right
(or both
as you will) on your #child
sibling to restore the normal document flow.