How to reliably distinguish on the client side from a mobile phone they are watching a site or from a desktop?

Question

Browsers

I want to set the font size in CSS, depending on whether they are watching from a mobile phone or from a laptop.
To navigate the screen resolution is such a thing, in this case.
What else can be done on the client side? (the backing is not supposed at all yet)
Is it possible somehow to resolve all this using pure CSS, without the help of JS?
Thank you.

Answer 1

https://vanseodesign.com/css/media-queries/

Answer 2

Try something like this hgoebl.github.io/mobile-detect.js

Answer 3

Is the user agent that defines Android / iPhone not suitable?
www.useragentstring.com/pages/useragentstring.php

<?php
$iPod    = stripos($_SERVER['HTTP_USER_AGENT'],"iPod");
$iPhone  = stripos($_SERVER['HTTP_USER_AGENT'],"iPhone");
$iPad    = stripos($_SERVER['HTTP_USER_AGENT'],"iPad");
$Android = stripos($_SERVER['HTTP_USER_AGENT'],"Android");
if( $iPod || $iPhone || $iPad ){
    echo "ios";
}else if($Android){
    echo "android";
}

Although, so-so solution.
I love media queries, em fonts.