Comment # 2 on bug 1175107 from Dr. Werner Fink

You might also miss

  auto_view text/html

in your personal ~/.muttrc ... or likewise

  alternativeview_order text/plain text/html

or

  alternativeview_order text/html text/plain

or

  set implicit_autoview=yes

last one is a bad idea as it might follow bad links

Also otherwise F1 is you friend within mutt  ... also don't foget
`copiousoutput` ... see manual page mailcap(4)