These "font-size" classes are absolute in visual size. This means that regardless of whether your html tag is a <h1> – <h6>, <p>, etc... the font-size of these elements will be identical once the class is applied.
This font-size is 1.
<p class="fs-1">Text</p>
This font-size is 2.
<p class="fs-2">Text</p>
This font-size is 3.
<p class="fs-3">Text</p>
This font-size is 4.
<p class="fs-4">Text</p>
This font-size is 5.
<p class="fs-5">Text</p>
This font-size is 6.
<p class="fs-6">Text</p>
This font-size is 7.
<p class="fs-7">Text</p>
This font-size is 8.
<p class="fs-8">Text</p>
This font-size is 9.
<p class="fs-9">Text</p>