A v. NU HTML checker flymake backend.
Ensure that you have java installed and download the NU HTML checker. Then, customize the flymake-vnu-jar
variable to the location of the downloaded vnu.jar file.
(require 'flymake-vnu)
(with-eval-after-load 'flymake
(flymake-vnu-setup))
(use-package flymake-vnu
:ensure t
:config
(flymake-vnu-setup))