Atlassian has developed AUI, the Atlassian User Interface. This is a set of UI components and a front end library do develop applications according to the Atlassian Design Guidelines. As of 2019-11, version 8.5.1 is the most recent version.
A second component to build apps on Atlassian is ACE, or Atlassian Connect Express, a framework in javascript. This framework currently uses an outdated ACE version by default. In the head of the html delivered by ACE, AUI 5.8.12 is used:
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="ap-local-base-url" content="{{localBaseUrl}}">
<title>{{title}}</title>
<link rel="stylesheet" href="//aui-cdn.atlassian.com/aui-adg/5.8.12/css/aui.css" media="all">
<link rel="stylesheet" href="//aui-cdn.atlassian.com/aui-adg/5.8.12/css/aui-experimental.css" media="all">
<!--[if IE 9]><link rel="stylesheet" href="//aui-cdn.atlassian.com/aui-adg/5.8.12/css/aui-ie9.css" media="all"><![endif]-->
<link rel="stylesheet" href="/css/addon.css" type="text/css" />
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/5.8.12/js/aui-soy.js" type="text/javascript"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/5.8.12/js/aui.js" type="text/javascript"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/5.8.12/js/aui-datepicker.js"></script>
<script src="//aui-cdn.atlassian.com/aui-adg/5.8.12/js/aui-experimental.js"></script>
<script src="https://connect-cdn.atl-paas.net/all.js" type="text/javascript"></script>
The update to AUI 8.5.1 is not described in the documentation. Here is the header you need to use:
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="ap-local-base-url" content="{{localBaseUrl}}">
<title>{{title}}</title>
<script src="{{hostScriptUrl}}" type="text/javascript"></script>
<link rel="stylesheet" type="text/css" href="https://unpkg.com/@atlassian/aui@8.5.1/dist/aui/aui-prototyping.css"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js"></script>
<script src="https://unpkg.com/@atlassian/aui@8.5.1/dist/aui/aui-css-deprecations.js"></script>
<script src="https://unpkg.com/@atlassian/aui@8.5.1/dist/aui/aui-prototyping.js"></script>
<link rel="stylesheet" href="/css/addon.css" type="text/css" />It’s simple once you know what to do.