CSS fonts rem trick: 62.5% or 6.25%

Question

I would like to use font sizing with REM and on internet I found following trick:

html { font-size: 62.5%; } 
body { font-size: 1.4rem; } /* =14px */
h1   {          


        

Answer 1

62.5% of 16px is 10px, a much more reasonable base font size that can serve as a fallback for older browsers that don't support the rem unit such as IE8, Firefox 3.5, Safari 4 and Opera 11. If you set font-size: 6.25%, older browsers that don't understand rems will ignore all your rem declarations and see your entire site in the same very small print, making it unreadable. Keep in mind that 6.25% of 16px (user-defined font sizes notwithstanding) is one pixel.

There has been nothing wrong with the traditional approach and there are no benefits to deviating from it the way you have — only pitfalls. Yes, you can say that you're not supporting older browsers, and that's fine, but that doesn't change the fact that someone who happens across your site in an older browser is going to get an unreadable experience that wouldn't happen with the traditional approach just because you, the author, wanted 1:1 px-to-rem mappings in your stylesheet.

Answer 2

You can also use pixels - the default font-size is 16px by default, setting 10px as a base allows you to use rems whereby 1rem = 10px because the root is set as 10px.

html { font-size: 10px}
body { font-size: 1.4rem; } /* =14px */
h1   { font-size: 2.4rem; } /* =24px */

Answer 3

You could use 6.25% so that you get your nice 1:1 rem values, then set font-size: 16em; on body to get the best of both worlds. It'll fix the tiny font problem in old browsers, and browsers that do understand your rem declarations will ignore it by looking to html's font-size when calculating sizes. As far as I can tell there are no drawbacks to doing it this way and you actually get a better sized fallback than you do with 62.5%, but there may be something I've missed so use it with caution.