Good practice 1: use relative sizing values.
The use of em, rem, and % is preferable.
Good practice 2: set the root element to 100%.
By setting the font size of the html root element we maintain the browser's default size and we can start working with relative sizing. The rem value will be relative to the root: the <html> element.
html {
font-size: 100%;
}
/* Alternativelly, this can be applied to the <body> element */