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$("dd:first").css("color","red"); //最初のddの文字色を赤く
$(".box dt:first-child").css("text-decoration","underline");　//最初のdtをそれぞれ下線
$("dd:first-child").css("font-weight","bold"); //最初のddをそれぞれ太字にできません
$(".box dd:first-of-type").css("background","#fc3");　//最初のddをそれぞれ背景黄色に

<div class="box">
    <h1>Header</h1>
    <p>Paragraphparagraph</p>
    <dl>
        <dt>dt</dt><dd class="text">dd</dd>
        <dt>dt</dt><dd class="text">dd</dd>
        <dt>dt</dt><dd class="text">dd</dd>
    </dl>
</div>
<div class="box">
    <h1>Header</h1>
    <p>Paragraphparagraph</p>
    <dl>
        <dt>dt</dt><dd class="text">dd</dd>
        <dt>dt</dt><dd class="text">dd</dd>
        <dt>dt</dt><dd class="text">dd</dd>
    </dl>
</div>
<div class="box">
    <h1>Header</h1>
    <p>Paragraphparagraph</p>
    <dl>
        <dt>dt</dt><dd class="text">dd</dd>
        <dt>dt</dt><dd class="text">dd</dd>
        <dt>dt</dt><dd class="text">dd</dd>
    </dl>
</div>

dl dd:first-child { background: #fc3; }

.box { border: #e4e4e4 1px solid; width: 150px; padding: 10px; float: left; margin: 0 15px 15px 0; }
dt { clear: left; float: left; width: 2em; }
dd { margin-left: 3em; }