How do I select the first div in these divs (the one with id=div1) using first child selectors?

<div class="alldivs">
   <div class="onediv" id="div1"> 1 This one </div>
   <div class="onediv" id="div2"> 2 </div>
   <div class="onediv" id="div3"> 3 </div>
</div>
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