Enable prism-whitespace-mode automatically for XML and YAML files, and detect amount of whitespace per indentation level:
(use-package prism
:config
(add-to-list 'prism-whitespace-mode-indents '(yaml-mode . yaml-indent-offset))
(add-to-list 'prism-whitespace-mode-indents '(nxml-mode . nxml-child-indent))
:hook
(yaml-mode . prism-whitespace-mode)
(nxml-mode . prism-whitespace-mode))
As usual, embed accordingly in your own init.el.
#emacs