The interwebs are full of ancient doctrines for folding code in the Glorious Emacs Editor. Among them are folding.el, outline-minor-mode, and fold-dwim.el. Alas, my poor unenlightened mind cannot write the necessary elisp configuration.
Please help a mortal fold tags in nxml-mode, preferably with TAB. If relevant, I use viper and vimpulse.