What is the base tag and what does it do?

The <base> tag specifies the default address or default target for all links on a page.
Normally, browsers fill in the blanks in relative URLs by extracting the corresponding elements from the URL of the current document.
Use the <base> tag to change this. The browser will then resolve all relative URLs using the specified base URL instead of the current document's URL. This includes URLs in <a>, <img>, <link>, and <form> tags.

test:

1: The link path placed before the base tag can use a relative path and is relative to the current path.
However, the link placed after it using a relative path will be relative to the path specified by base.
2: After using the base tag, the relative path of the link in the body will be relative to the path specified by base.
3: After using the base default target, the subsequent opening method will be opened in the method specified by base, unless the opening method is explicitly defined later.
Summary: base actually defines a default address and default target. All subsequent link elements will work.