Comments on Bootstrap 4.3.1 Template File

The basic Bootstrap version 4.3.1 html file found at https://getbootstrap.com/docs/4.3/getting-started/introduction/#starter-template is shown below.

<!doctype html>
<html lang="en">
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<!-- Bootstrap CSS -->
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" 
  integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
<title>Hello, world!</title>
</head>
<body>
<h1>Hello, world!</h1>
<!-- Optional JavaScript -->
<!-- jQuery first, then Popper.js, then Bootstrap JS -->
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" 
   crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" 
   integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" 
   integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
</body>
</html>

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The reference to the Bootstrap CSS file is placed in the head so that when the html in the body section of the page is loaded, it can be immediately styled by this CSS file.
The reference to the Bootstrap CSS file is to the "minified" version of the file. This is the CSS file with unnecessary spaces removed so as to reduce the loading time for the file. The downside is that the file is almost impossible to read. So when you are developing your page, you may want to reference the non-minified version so that you can examine the CSS.
The use of a content delivery network (CDN) makes these files available to your page without having them to reside on your website. Also, if visitors to your web page have recently visited, using the same browser, other web sites that also reference these files on this CDN, the files will be available in the browser's cache and need not be downloaded.
The HTML5 shivs referenced in the Bootstrap 3 template file do not appear here because Bootstrap 4 does not support Internet Explorer 9 and before; if you need support for Internet Explorer 9 or before, use Bootstrap 3.
These three JavaScript file references are again to the minified version of these files.
The reference to the jQuery JavaScript framework is placed at the bottom of the page so that the loading of the content of the page is not delayed by the loading of the jQuery framework. This reference can either be to a CDN or to the desired version of jQuery from http://jquery.com/download/ that you download and and place within the [js] folder.
The order of the JavaScript files must be jQuery first, then popper (a JavaScript library to help with positioning HTML elements), and then the Bootstrap JavaScript files.

This template file should be placed in your site folder. Your site folder should contain three folders [css], [fonts], and [js] at the top level of the site. Even if you are going to access Bootstrap and jQuery through a CDN, it is still a good idea to create these folders since you may want to supplement the Bootstrap css file with your own css file and have your own JavaScript file to supplement the Bootstrap JavaScript file.

If you are properly connected to Bootstrap and the internet (if accessing Bootstrap via a CDN), when you view the template file page in the browser the following will be true:

The text "Hello World" in the body will appear in a sans-serif font; in the absence of a font specification, browsers generally default to Times Roman or Times New Roman, which has serifs, and
The text "Hello World" is up againist the left margin

Revised: April 23, 2019