There is only one truth. It is the source.
November 7, 2011
The base tag defines a base URL for all relative URLs to have. On a Web page this is not too useful because the page's current URL works as the base URL, but in an email generated by the backend, it is invaluable since unlike a Web page, an email does not have a current URL. Without a base tag, including relative links in an email results in links that do not work.
Consider the code:
<base href="https://mydomain.com" /> <a href="/relative_thing/">Click me</a>
Clicking the anchor directs you to https://mydomain.com/relative_thing/. This means that generated emails may contain relative URLs (Django's template language generates URLs relatively) without issue.