Chrome, FF, Opera, takie same.
<p class='a'>lorem</p>
<p class='b'>lorem</p>
<pre></pre>
* {
box-sizing: border-box;
margin: 20px 60px;
padding: 0;
}
html {
font-size: 62.5%; /* 10px */
}
body {
font-size: 1.6rem; /* 16px */
font-family: sans ;
}
.a { font-size: 40px; }
.b { font-size: 4rem; }
const pre = document.querySelector('pre'),
a = document.querySelector('.a'),
b = document.querySelector('.b');
window.addEventListener('DOMContentLoaded', () => {
pre.textContent = 'a: ' + window.getComputedStyle(a).getPropertyValue('font-size') + '\n'
+ 'b: ' + window.getComputedStyle(b).getPropertyValue('font-size');
});