revert

h3 {
  font-weight: normal;
  color: blue;
}

<h3 >
  This will still have font-weight: normal, but color: black
</h3>
<p>Just some text</p>
<h3 >
  This should have its original font-weight (bold) and color: black
</h3>
<p>Just some text</p>

h3 {
  font-weight: normal;
  color: blue;
  border-bottom: 1px solid grey;
}

<h3>This will have custom styles</h3>
<p>Just some text</p>
<h3 >This should be reverted to browser/user defaults</h3>
<p>Just some text</p>

section {
  color: darkgreen;
}
p {
  color: red;
}
section.with-revert {
  color: revert;
}

<section>
  <h3>   h3  </h3>
  <p>paragraph  </p>
      .
</section>
<section class="with-revert">
  <h3>revert    h3 </h3>
  <p> paragraph  </p>
     revert  
</section>