Question

With this css

.addProblemClass{
    width:300px;
    height:300px;
    /*width:25%;
    height:40%;*/
    border:solid 1px #000000;
    margin: 5px;
    background-color:#FFFFFF;
    padding:5px;
    opacity:0.9;/*For chrome and mozilla*/
    filter:alpha(opacity=90);/*For IE*/
}

.boxHeader{
    border: solid 1px #000000;
    height: 20%;
    padding: 5px;
}

.addProblemHeaderTextDiv{
    border:solid 1px #FF0000;
    width: 80%;
    height: 100%;
}

.addProblemHeaderImageDiv{
    border:solid 1px #00FF00;
    float: left;
    width: 20%;
    height: 100%;
}

and this html

<div class="addProblemClass">
            <div class="boxHeader">
                <div class="addProblemHeaderImageDiv"></div>//DIV A
                <div class="addProblemHeaderTextDiv"></div>//DIV B
            </div>
        </div>

why DIV A and DIV B are overllaping?

Answer 1

You cant do this because of the CSS Box model.. it adds the 1px border like this

20% + 80% = 100% width + 1px border 

This could work, by subtracting the border again with margin. Else you must use more markup i am afraid.

.addProblemHeaderTextDiv{
    border:solid 1px #FF0000;
    width: 80%;
    margin: 0 -1px;
    height: 100%;
    float: left;

}

.addProblemHeaderImageDiv{
    border:solid 1px #00FF00;
    margin: 0 -1px;
    float: left;
    width: 20%;
    height: 100%;
}