Based on Google's tpl-mode for ctemplates, this major mode adds support for Mustache's more lenient tag values and includes a few commands for your editing pleasure.

In your shell:

cd ~/.emacs.d/vendor
curl --location -O https://github.com/mustache/emacs/raw/master/mustache-mode.el

In your Emacs config:

(add-to-list 'load-path "~/.emacs.d/vendor/mustache-mode.el")
(require 'mustache-mode)