Every browser that supports the double colon (::) CSS3 syntax also supports just the (:) syntax, but IE 8 only supports the single-colon, so for now, it's recommended to just use the single-colon for best browser support.
:: is the newer format indented to distinguish pseudo content from pseudo selectors. If you don't need IE 8 support, feel free to use the double-colon.
:: 是新的形式，目的是区分伪类内容从伪类选择器中。如果你不需要IE8的支持，可以随意使用双冒号。