title: Why does Margin Collapsing only apply to top and bottom but not left and right
body: <p>In CSS, what is the motivation for <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Box_Model/Mastering_margin_collapsing" rel="nofollow">margin-collapsing</a> behavior only applying between top and bottom margins but not left and right?</p>