问题
-webkit-margin-{before,after,start,end} can be overriden by margin-{top,bottom,left,right} respectively. Why does Chrome’s user agent stylesheet not just use the standard margin- declarations?
回答1:
This is to do with layout direction.
So -webkit-margin-start is like margin-left when the layout direction is left-to-right, but like margin-right when the direction is right-to-left.
See http://jsfiddle.net/jLnz2jd1/ (In Chrome, obviously)
In contexts where directionality matters, the user-agent values for these properties can be overridden. Where it doesn't matter the fallback to the standard margin properties is automatic.
来源：https://stackoverflow.com/questions/27707576/in-chrome-s-user-agent-stylesheet-why-use-webkit-margin-before-et-al-instead