content-box

https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing

http://www.w3school.com.cn/cssref/pr_box-sizing.asp

http://www.w3school.com.cn/tiy/t.asp?f=css3_box-sizing

http://blog.sina.com.cn/s/blog_877284510101kt87.html