Units REM. What to use?

Question

Browsers

I read about Rem (root em). And its support for browsers. Why, for example, REM for Internet explorera (He doesn't have a mobile Ei? ) . Another question: if you use pixels first for the folback and then rem, then in opera mini the fonts will be too large or they will scala themselves. And the latest, which browsers can automatically change the font sizes specified in px? Why then use relative units?

Answer 1

He doesn't have a mobile Ei, does he?

Windows Phone, Windows 10...
Actually, the whole point is that you set the font sizes globally, for blocks, and then with the help of rem - relative where necessary.

Answer 2

Timeline
https://www.google.ru/search?newwindow=1&q=timeline+css