styles/mixins/common.less

.placeholder(@color: @input-placeholder-color) {
    // Firefox
    &::-moz-placeholder {
        color: @color;
        opacity: 1; // Override Firefox's unusual default opacity; see https://github.com/twbs/bootstrap/pull/11526
    }
    // Internet Explorer 10+
    &:-ms-input-placeholder {
        color: @color;
    }
    // Safari and Chrome
    &::-webkit-input-placeholder {
        color: @color;
    }
}