The Configuration and ConfigurationImpl classes are poorly named and misleading. They imply interface and implementation, which is incorrect, in the larger design of the standard doclet.

The package for Configuration gives a hint as to its true nature:

jdk.javadoc.internal.doclets.formats.html

It is the extended configuration for HTML-formatted output.

ConfigurationImpl should be auto-renamed to HtmlConfiguration

As a secondary change, suggest:

Configuration should be auto-renamed to BaseConfiguration or BasicConfiguration.

This is just a recommendation to do an IDE-rename of the types. I do not recommend changing the instance variable names. (which are typically all just called "configuration" anyway)