default classes

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNameBindings: ['defaultClasses'],
  init() {
    this._super(...arguments);
    let providedClasses = this.get('class');
    
    // if we don't find a certain class
    // we add default classes
    if (!providedClasses || providedClasses.indexOf('b') === -1) {
      this.set('defaultClasses', 'a another-class');
    }
  }
});

controllers.application.js

import Ember from 'ember';

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

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.a.b {
  color: red;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{#my-component class="b"}}
class="b"
{{/my-component}}

{{#my-component}}
no class
{{/my-component}}
<br>
<br>

templates.components.my-component.hbs

{{yield}}

twiddle.json

{
  "version": "0.7.2",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.4/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.4.3/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.4/ember-template-compiler.js"
  }
}