shr is an Emacs-native, text-based HTML-renderer, which is tightly
integrated with the message viewer.
It has been the default option in mu4e for quite some time. Overall, it is a
good option for viewing HTML if it is able to do so, and well-integrated with
Emacs. It has its limitations though, and some HTML e-mail messages may not be
supported very well.
If you use a dark theme in Emacs, you may need to tweak shr to make the
messages better readable:
(setq shr-color-visible-luminance-min 80)