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<section id="GroupA" class="group">
    <h3>Group A</h3>
    <div id="GroupASub1" class="subgroup">
        <h4>Group A Sub 1</h4>
    </div>
    <div id="GroupASub2" class="subgroup">
        <h4>Group A Sub 2</h4>
    </div>
</section>
<section id="GroupB" class="group">
    <h3>Group B</h3>
    <div id="GroupBSub1" class="subgroup">
        <h4>Group B Sub 1</h4>
    </div>
    <div id="GroupBSub2" class="subgroup">
        <h4>Group B Sub 2</h4>
    </div>
</section>
<section id="GroupC" class="group">
    <h3>Group C</h3>
    <div id="GroupCSub1" class="subgroup">
        <h4>Group C Sub 1</h4>
    </div>
    <div id="GroupCSub2" class="subgroup">
        <h4>Group C Sub 2</h4>
    </div>
</section>    

.group {
    background: yellow;
    width: 200px;
    height: 500px;
}
.group .subgroup {
    background: orange;
    width: 150px;
    height: 200px;
}