These two scripts are for formatting html/xml in RSpec tests (presumably for exploration and debugging). They are intended to be temporarily copied into the lib directory when needed instead of installed as a gem because sometimes when debugging somebody else's project (or even your own), it's easier not to have to add items to the Gemfile. That's especially true if you've already made edits to the Gemfile that you don't want to interfere with.
Just copy either script into the lib directory of your rails app.
Make sure to require the appropriate script in your test file. For example:
require 'format_html_rexml'. You will then have access to the methods
display_formatted_html and formatted_html which take a single string
argument containing html. The former will ouput formatted html while the latter
will simply return it. If you are using Capybara, outputting the current page's
html looks like this: display_formatted_html(page.html).
The advantage of using format_html_rexml.rb is that is uses REXML which is included in the standard library. So, if you are using Bundler, you won't get a complaint that REXML isn't in your Gemfile. It also won't include an xml header like the format_html_nokogiri.rb version will. REXML's parsing ability is not as robust as Nokogiri's though. Minor html syntax errors will cause an error and stop the html from being formatted. If you are looking for html syntax errors, that could be an advantage though.