gulp-clean-css底层用的是clean-css,更多参数可以看clean-css的内容。关于这个参数： 其文档也有说明>>For example, this declaration --compatibility 'ie8,+units.rem' will ensure IE8 compatiblity while enabling rem units so the following style margin:0px 0rem can be shortened to margin:0, while in pure IE8 mode it can't be.