In a Single Page Application, the container is the object in which the application is initially loaded and displayed to the user. This concept is different from that of a software container, which is an isolated environment that an application lives in, much like a virtual machine.
For a single page web application, the container could be the <body> element, or any element within the <body> element. For instance, you may have some static welcome text initially loaded on the page in a <p> element, and the SPA will then load dynamically below that element in a <div> element:
<!doctype html>
<html>
<body>
<p>This is some welcome text.</p>
<div class="container">
The application will be loaded here.
</div>
</body>
</html>
In a scenario like this, you will always have some fixed content...