rem is normal

This is normal, declare your font-size, inherit, tweak and relax.

HTML

<ul>
    <li>this is parent
        <ul>
            <li>this is child
                <ul>
                    <li>this is grandchild
                        <ul>
                            <li>this is great-grandchild</li>
                        </ul>
                    </li>
                </ul>
            </li>
        </ul>
    </li>
</ul>

CSS

li {

      font-size: 2rem;

  }