As Adrian Sandu explains in https://www.sitepoint.com/understanding-and-using-rem-units-in-css/
font-size: 62.5% should be an standard in the html
css
html { font-size: 62.5%; } /* =10px */
body { font-size: 1.4rem; } /* =14px */
h1 { font-size: 2.4rem; } /* =24px */