CSS Mixin

I made a CSS (pre?)processor that runs client-side. It is very efficient, because rather than duplicating rules, it updates selectors using the CSSOM. It probably has bad error handling, so please try to break it so I can make it better.

This page's CSS:

[--mixin="blue-italic"]{
  font-style: italic;
  color: #373fff;
}

[--mixin="round-border"]{
  border:1px black solid;
  border-radius:0.5rem;
  padding:0.5rem;
}

h1 {
  --mixins:blue-italic;
}

pre{
  --mixins:blue-italic round-border;
}
body{
  --mixins:round-border;
}

Made on Glitch!

Some more blue-italic stuff.