src/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;
    }
}
.user-select(@type: none) {
    -webkit-user-select: @type;
    -moz-user-select: @type;
    user-select: @type;
}
// for select and input like component's arrow
.inner-arrow() {
    position: absolute;
    top: 50%;
    right: 8px;
    line-height: 1;
    margin-top: -7px;
    font-size: @font-size-base;
    color: @subsidiary-color;
    .transition(all @transition-time @ease-in-out);
}