@angular/cdk/a11y/_index.scss

/// Emits a CSS class, `.cdk-visually-hidden`. This class can be applied to an element
/// to make that element visually hidden while remaining available to assistive technology.
@mixin a11y-visually-hidden() {
  .cdk-visually-hidden {
    border: 0;
    clip: rect(0 0 0 0);
    height: 1px;
    margin: -1px;
    overflow: hidden;
    padding: 0;
    position: absolute;
    width: 1px;

    // This works around a Chrome bug that can cause the tab to crash when large amounts of
    // non-English text get wrapped: https://bugs.chromium.org/p/chromium/issues/detail?id=1201444
    white-space: nowrap;

    // Avoid browsers rendering the focus ring in some cases.
    outline: 0;

    // Avoid some cases where the browser will still render the native controls (see #9049).
    -webkit-appearance: none;
    -moz-appearance: none;

    // We need at least one of top/bottom/left/right in order to prevent cases where the
    // absolute-positioned element is pushed down and can affect scrolling (see #24597).
    // `left` was chosen here, because it's the least likely to break overrides where the
    // element might have been positioned (e.g. `mat-checkbox`).
    left: 0;

    [dir='rtl'] & {
      left: auto;
      right: 0;
    }
  }
}

/// @deprecated Use `a11y-visually-hidden`.
@mixin a11y() {
  @include a11y-visually-hidden;
}

/// Emits the mixin's content nested under `$selector-context` if `$selector-context`
/// is non-empty.
/// @param {String} selector-context The selector under which to nest the mixin's content.
@mixin _optionally-nest-content($selector-context) {
  @if ($selector-context == '') {
    @content;
  }
  @else {
    #{$selector-context} {
      @content;
    }
  }
}

/// Applies styles for users in high contrast mode. Note that this only applies
/// to Microsoft browsers. Chrome can be included by checking for the `html[hc]`
/// attribute, however Chrome handles high contrast differently.
///
/// @param {String} target Type of high contrast setting to target. Defaults to `active`, can be
///     `white-on-black` or `black-on-white`.
/// @param {String} encapsulation Whether to emit styles for view encapsulation. Values are:
///     * `on` - works for `Emulated`, `Native`, and `ShadowDom`
///     * `off` - works for `None`
///     * `any` - works for all encapsulation modes by emitting the CSS twice (default).
@mixin high-contrast($target: active, $encapsulation: 'any') {
  @if ($target != 'active' and $target != 'black-on-white' and $target != 'white-on-black') {
    @error 'Unknown cdk-high-contrast value "#{$target}" provided. ' +
           'Allowed values are "active", "black-on-white", and "white-on-black"';
  }

  @if ($encapsulation != 'on' and $encapsulation != 'off' and $encapsulation != 'any') {
    @error 'Unknown cdk-high-contrast encapsulation "#{$encapsulation}" provided. ' +
           'Allowed values are "on", "off", and "any"';
  }

  // If the selector context has multiple parts, such as `.section, .region`, just doing
  // `.cdk-high-contrast-xxx #{&}` will only apply the parent selector to the first part of the
  // context. We address this by nesting the selector context under .cdk-high-contrast.
  @at-root {
    $selector-context: #{&};

    @if ($encapsulation != 'on') {
      // Note that if this selector is updated, the same change has to be made inside
      // `_overlay.scss` which can't depend on this mixin due to some infrastructure limitations.
      .cdk-high-contrast-#{$target} {
        @include _optionally-nest-content($selector-context) {
          @content;
        }
      }
    }

    @if ($encapsulation != 'off') {
      .cdk-high-contrast-#{$target} :host {
        @include _optionally-nest-content($selector-context) {
          @content;
        }
      }
    }
  }
}