The :host selector targets an Angular component's host element. Angular view encapsulation requires it when styles must respond to an attribute applied to the component itself rather than only to elements inside its template. section 28.3.2
Styling a disabled custom button
A disabled customButton component can style its internal button by combining :host[disabled] with the button's :disabled state. This supports both disabling the host component and disabling the native button. section 28.3.2
...
:host[disabled] button[class*="custom-button-"],
button[class*="custom-button-"]:disabled {
background-color: material.get-color-from-palette($bg, disabled-button);
color: material.get-color-from-palette($fg, disabled-button);
}
...
Inspect the SCSS selector that applies Angular Material disabled-button foreground and background colors when either the component host or internal button is disabled. section 28.3.2
