On some platforms certain gems have a dependency on system libraries. This is usually done for performance reasons. Ruby is an interpreted language so tasks, such as parsing XML can be slow; therefore, it makes sense to delegate that task to a system library.
On Windows you won't need to worry about this, though you will have to ensure you have the Ruby DevKit installed; see http://rubyinstaller.org/add-ons/devkit for detailed instructions on how to do this.
Capybara has a dependency on Nokogiri, the popular Ruby-based XML parser. This in turn needs the following system libraries to be available:
libxml2
libxml2-dev
libxslt
libxslt-dev
Tip
The latest version of Nokogiri now includes these dependencies within the gem itself. It is still worth installing the system libraries globally, however, as you will surely encounter projects that rely on versions of Nokogiri prior to 1.6.0.
How you install these on a particular system will differ, for example, apt-get
for Ubuntu, yum
for Red Hat, or brew
for Mac OS X.