force a div to contain floated child divs

I’m trying to customize the twentyeleven theme in WordPress.
It has a 2-columns layout set up by:

<div id="main">
     <div id="primary"></div>
     <div id="secondary"></div>
</div>

and

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#main {
    clear: both;
    padding: 1.625em 0 0;
}
#primary {
    float: left;
    margin: 0;
    width: 100%;
}
#secondary {
    float: right;
    margin-right: 7.6%;
    width: 18.8%;
}

I am not sure that there are other relevant css properties inside style.css which I have not recognized. Anyway, the child divs lie outside #main. How can I force to have the child divs contained in #main? (apart from reducing the width of #primary, which yields no effect to me)

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4 comments

  1. You should use the clearfix method with pseudo-element :after because in your case, the clear isn’t really applied after the children.

    #main:after {
        content: ".";
        display: block;
        clear: both;
        visibility: hidden;
        line-height: 0;
        height: 0; 
    }
    

    http://jsfiddle.net/zfsjb/

    EDIT 2020:

    Since a while now, this simpler solution works just as fine:

    #main::after {
        content: "";
        display: table;
        clear: both;
    }
    

    Also it’s now recommended to use ::after instead of :after to match other pseudo-elements.