node_modules/element-ui/packages/theme-chalk/src/display.scss

@import "common/var";
@import "mixins/mixins";
.hidden {
  @each $break-point-name, $value in $--breakpoints-spec {
    &-#{$break-point-name} {
      @include res($break-point-name, $--breakpoints-spec) {
        display: none !important;
      }
    }
  }
}