How can I adjust text font size?

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I just want the font size to be smaller on smaller screens. But I notice that "fs-6" is always 16px and "fs-5" is always 20px no matter how big the viewport is, but "fs-1" ~"fs-4" seem to be responsive when the viewport is changed. Aren't they all supposed to be responsive? And it doesn't sound good that the smallest size "fs-6" is 16px which is already very big.

I have tried adding breakpoints to "fs" to make it responsive like the following, but it doesn't work at all (Bootstrap v5.0.2):

<p class="fs-1 fs-md-3">Test text</p>

It seems that adding breakpoints to "fs" is not allowed, so "fs-md-3" cannot be recognized. Could anyone tell me a method to display smaller font size on smaller screens? I don't need smooth transitions like using RFS. Thank you very much!