Additive class vs. classNames

components.bad-parent-component.js

import Ember from 'ember';

export default Ember.Component.extend({
});

components.good-parent-component.js

import Ember from 'ember';

export default Ember.Component.extend({
});

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: ['base-class'],
  style: 'border: solid 2px blue'
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

styles.app.css

.base-class {
  border: solid 3px blue;
  padding: 20px;
  margin-bottom: 20px;
}

.outer-class {
  box-shadow: inset 0 0 0 3px red; 
}

.from-parent {
  color: green;
}

templates.application.hbs

{{my-component classNames='outer-class'}}

{{my-component class='outer-class'}}

{{component 'my-component' classNames='outer-class'}}

{{component 'my-component' class='outer-class'}}

{{bad-parent-component child=(component "my-component" class="outer-class")}}

{{good-parent-component child=(component "my-component" class="outer-class")}}

templates.components.bad-parent-component.hbs

{{component child class="from-parent"}}

templates.components.good-parent-component.hbs

{{component child classNames="from-parent"}}

templates.components.my-component.hbs

I am a component

twiddle.json

{
  "version": "0.11.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.4.5",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.4.5",
    "ember-testing": "2.4.5"
  },
  "addons": {}
}