I found the bug looking at the Github "bug" badge that appears on issues and isolated it to this snippet:

<style>
  span {
    color: hsl(0deg, 0%, calc(1 * 100%));

    /* This works:  */
    /* color: hsl(0deg, 0%, 100%); */

    /* While this produces the "bug" in FF too: */
    /* color: hsl(0deg, 0%, 100); */

    background-color: red;
    font-size: 10em;
  }
</style>
<span> bug </span>

This is how it looks in Firefox:

...and this is how it looks in Ladybird:

I think that the problem is that calc doesn't preserve the % in the result.

I was also able to spot a FIXME in StyleValue.cpp about calc and persentages but I'm not sure on how to proceed further since my C++ isn't that great..

First issue on Serenity, I hope I've been doing things right and that this will be helpful to someone :^)