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Negative margins don't hold position of elements after it and alter the flow of everything following.
<br>
<br>
<div class="div1">
<div class="div2">test</div>
<div class="div3">stuff</div>
</div>
<br>
Relative positioning with negative top holds the position of elements after it and alter the flow of everything following.
<br>
<br>
<div class="div1">
<div class="div5">test</div>
<div class="div3">stuff</div>
</div>