CSS transition demo

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  activeClass: 'faded-in',
  
  actions: {
    toggleActiveClass() {
      if (this.activeClass === 'faded-in') {
        this.set('activeClass', 'faded-out');
      } else {
        this.set('activeClass', 'faded-in');
      }
    }
  }
});

styles.app.css

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

.faded-in {
  opacity: 1;
}

.faded-out {
  opacity: 0;
}

.transition {
  transition: opacity 1s;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<button {{action 'toggleActiveClass'}}>Click me</button>
<div class={{concat 'transition ' activeClass}}>
  <p>Lorem ipsum dolor</p>
</div>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}