Now the majority of the sites are domestic use css set the Font size to 12 px, although the relative size of pixel units, but only in relation to the resolution of the screen when a certain screen resolution, the word if you are too small to read difficulties in the non-standard browser on the powerless, so that if the unit into em, then they can use CTRL rolling the mouse to enlarge the word, or use the View menu> Text Size to change the size of the word. 

  How to configure the 

  1. body labels, will be 62.5% as a font size, if not in body, CSS settings, the browser's default size is 16 px, and the establishment of 62.5%, Font Size = 16 = 62.5% px * 10px, then You can easily set up em size, such as 12 px you need, as long as a 1.2 em. 

   Body (font-size: 62.5%;) h1 (fong-size: 1.4em; / * h1 word equivalent to the size of 14 px * /)  

  2.body Set in the Font Size 75%, 16 font size = px * 75% = 12px, 1em equivalent to 12 px. 

   Body (font-size: 75%;) h1 (fong-size: 1.5em; / * h1 word equivalent to the size of 18 px * /)  

  The first method is better. 

  If the site is in English, also can be set to small, x-small ...... 

  Of course this way, the page design requirements will be higher.