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@content指令：
@mixin placeholder { ::-webkit-input-placeholder {@content} :-moz-placeholder{@content} ::-moz-placeholder {@content} :-ms-input-placeholder {@content} }@include placeholder { font-style:italic; color: white; font-weight:100;}从Sass 3.4开始，此mixin可以这样编写，以使其既可以嵌套也可以嵌套：
@mixin optional-at-root($sel) { @at-root #{if(not &, $sel, selector-append(&, $sel))} { @content; }}@mixin placeholder { @include optional-at-root('::-webkit-input-placeholder') { @content; } @include optional-at-root(':-moz-placeholder') { @content; } @include optional-at-root('::-moz-placeholder') { @content; } @include optional-at-root(':-ms-input-placeholder') { @content; }}用法：
.foo { @include placeholder { color: green; }}@include placeholder { color: red;}输出：
.foo::-webkit-input-placeholder { color: green;}.foo:-moz-placeholder { color: green;}.foo::-moz-placeholder { color: green;}.foo:-ms-input-placeholder { color: green;}::-webkit-input-placeholder { color: red;}:-moz-placeholder { color: red;}::-moz-placeholder { color: red;}:-ms-input-placeholder { color: red;}
