Install lynx (or links or w3m). Add this to your ~/.muttrc:
auto_view text/html
alternative_order text/plain text/html
Add this to ~/.mailcap:
text/html; lynx -dump -localhost -force_html %s | sed 's/^ *//'; copiousoutput
auto_view text/html
alternative_order text/plain text/html
Add this to ~/.mailcap:
text/html; lynx -dump -localhost -force_html %s | sed 's/^ *//'; copiousoutput
pandoc -f html -t plain
If you're going to use w3m, it's wise to wrap it with socksify to prevent email spam from tracking you:
export SOCKS_SERVER="127.0.0.1:1"
exec socksify w3m -dump text/html; unshare -U -n lynx -assume_charset=%{charset} -stdin -dump; copiousoutput;
(requires unprivileged userns, which is default enabled on recent distros. "sysctl -w kernel.unprivileged_userns_clone=1" if not.)