HTML root folder?

It is just a single character:

/

For example:

/image.jpg

whatever page you put it in, it refers to the same file, root_folder/image.jpg

You can also use <base> to set the base url all urls should be relative to. In general using an absolute path with / is probably prefereable, but <base> may be useful if you have the "root" of your page not on the root of the domain. I.E. example.com/mysite/index.html vs. example.com

https://developer.mozilla.org/en-US/docs/HTML/Element/base

You can just use full Url, like:

http://domain.com/file_in_root_folder.jpg

or absolute paths

/file_in_root_folder.jpg