The MicroProfile Config spec, the smallrye implementation of the MicroProfile Config spec, and the wildfly-microprofile-config based on the smallrye implementation include about a half dozen ConfigSources, from system properties to the wildfly microprofile-config-smallrye subsystem configuration.

None of these, however, accesses the ServletConfig and ServletContext parameters in web.xml files. These would seem to be worth adding as ConfigSources.