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It always takes me some time to choose a proper font-size, especially when I use rem to set element's width. I write css according to "mobile first". I want just to rewrite the html's font-size to change others width. It looks fine on the phone, however it looks not perfect on the desktop. So that I have to rewrite margin or padding on the desktop. Can you give me some tips to choose font-size between different devices? Thanks very much!