With an "after" element, note that between the first and the second examples, the model box of the div changes (when adding a border to 2, the margins of H1 and P are respected with respect to the DIV), between 3 and On 4 there is a BR to separate. A 3, 4 and 5 transparent border is applied to the DIV.
.contenedor{
width: 400px;
position:relative;
background-color: #eee;
}
.contenedor.bordealamitad::after{
position:absolute;
top:-1px;
left:0;
width:50%;
border-top: 2px solid #000;
content:" ";
}
.masancho {
width: 600px;
}
.borderesaltado {
border: 1px dashed red;
}
.bordetransparente {
border: 1px solid transparent;
}
<div class="contenedor bordealamitad">
<h2>1 Lorem ipsum Lorem ipsumLorem ipsumLorem</h2>
<p>Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem</p>
</div>
<div class="contenedor bordealamitad masancho borderesaltado">
<h2>2 Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem </h2>
<p>Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLoremLorem ipsum Lorem ipsumLorem ipsumLorem</p>
</div>
<div class="contenedor bordealamitad masancho bordetransparente">
<h2>3 Lorem ipsum Lorem ipsumLorem ipsumLorem </h2>
<p>Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem</p>
</div>
<br/>
<div class="contenedor bordealamitad bordetransparente">
<h2>4 Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem </h2>
<p>Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLoremLorem ipsum </p>
</div>
<div class="contenedor bordealamitad masancho bordetransparente">
<h2>5 Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem </h2>
<p>Lorem ipsum Lorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem ipsumLorem</p>
</div>