One question for the author: Why repurpose a Horizontal Rule <hr> when HTML has a perfectly functional Line Break tag <br>?
You can use @media queries to set display: none; on <br> tags and then those <br> tags have no effect, and then you get to decide when they kick in.
This is a great idea, but ignoring the built-in line break to repurpose and restyle the horizontal rule tag seems like an arbitrary choice for implementing the technique.