How and where to use ::ng-deep?

Usually /deep/ “shadow-piercing” combinator can be used to force a style down to child components. This selector had an alias >>> and now has another one called ::ng-deep.

since /deep/ combinator has been deprecated, it is recommended to use ::ng-deep

For example:

<div class="overview tab-pane" id="overview" role="tabpanel" [innerHTML]="project?.getContent( 'DETAILS')"></div>

and css

.overview {
    ::ng-deep {
        p {
            &:last-child {
                margin-bottom: 0;
            }
        }
    }
}

it will be applied to child components

USAGE

::ng-deep, >>> and /deep/ disable view encapsulation for specific CSS rules, in other words, it gives you access to DOM elements, which are not in your component's HTML. For example, if you're using Angular Material (or any other third-party library like this), some generated elements are outside of your component's area (such as dialog) and you can't access those elements directly or using a regular CSS way. If you want to change the styles of those elements, you can use one of those three things, for example:

::ng-deep .mat-dialog {
  /* styles here */
}

For now Angular team recommends making "deep" manipulations only with EMULATED view encapsulation.

DEPRECATION

"deep" manipulations are actually deprecated too, BUT it stills working for now, because Angular does pre-processing support (don't rush to refuse ::ng-deep today, take a look at deprecation practices first).

Anyway, before following this way, I recommend you to take a look at disabling view encapsulation approach (which is not ideal too, it allows your styles to leak into other components), but in some cases, it's a better way. If you decided to disable view encapsulation, it's strongly recommended to use specific classes to avoid CSS rules intersection, and finally, avoid a mess in your stylesheets. It's really easy to disable right in the component's .ts file:

@Component({
  selector: '',
  template: '',
  styles: [''],
  encapsulation: ViewEncapsulation.None  // Use to disable CSS Encapsulation for this component
})

You can find more info about the view encapsulation in this article.