https://www.thecodecampus.de/blog/angular-2-use-hostbindings-set-class/
And I played a little bit with it in the sandbox, it seems it can not only bind to class (class itself is just an attr) but also can bind a boolean value to a specific class as a switch. And it can bind to any customized attribute.
https://stackblitz.com/edit/angular-playground-325-fn6rkg?file=app%2Fbox%2Fbox.component.ts
in case forget about the syntax of using ampersand (&) in pseudo-class :host in sass/scss
here is the reference:
https://css-tricks.com/the-sass-ampersand/