advanced css splitter to deal with < IE 9 4095 selector bug. Via https://github.com/zweilove/css_splitter/blob/master/lib/css_splitter/splitter.rb and https://gist.github.com/ethanhinson/7521333

config.rb

# Requre a specific version in this file:
# gem 'zurb-foundation', '=4.3.1'
require 'zurb-foundation'

# Require any additional compass plugins here.

class Splitter
  
  MAX_SELECTORS_DEFAULT = 4095

  def self.split(infile, outdir = File.dirname(infile), max_selectors = MAX_SELECTORS_DEFAULT)

     raise "infile could not be found" unless File.exists? infile

     rules = IO.readlines(infile, "}")
     return if rules.first.nil?

     charset_statement, rules[0] = rules.first.partition(/^\@charset[^;]+;/)[1,2]
     return if rules.nil?

     # The infile remains the first file
     file_id = 1
     selectors_count = 0
     output = nil

     rules.each do |rule|
       rule_selectors_count = count_selectors_of_rule rule
       selectors_count += rule_selectors_count

       # Nothing happens until the selectors limit is reached for the first time
       if selectors_count > max_selectors
         # Close current file if there is already one
         output.close if output

         # Prepare next file
         file_id += 1
         filename = File.join(outdir, File.basename(infile, File.extname(infile)) + "_#{file_id.to_s}" +    File.extname(infile))
         output = File.new(filename, "w")
         output.write charset_statement

         # Reset count with current rule count
         selectors_count = rule_selectors_count
       end

       output.write rule if output
     end
  end
 
 # returns the specified split of the passed css_string
 def self.split_string(css_string, split = 1, max_selectors = MAX_SELECTORS_DEFAULT)
   rules = split_string_into_rules(css_string)
   extract_part rules, split, max_selectors
 end

 # splits string into array of rules (also strips comments)
 def self.split_string_into_rules(css_string)
   strip_comments(css_string).chomp.scan /[^}]*}/
 end
 
 # extracts the specified part of an overlong CSS string
 def self.extract_part(rules, part = 1, max_selectors = MAX_SELECTORS_DEFAULT)
   return if rules.first.nil?

   charset_statement, rules[0] = extract_charset(rules.first)
   return if rules.nil?

   output = charset_statement || ""
   selectors_count = 0
   selector_range = max_selectors * (part - 1) + 1 .. max_selectors * part # e.g (4096..8190)

   current_media = nil
   selectors_in_media = 0
   first_hit = true
   rules.each do |rule|
     media_part = extract_media(rule)
     if media_part
       current_media = media_part
       selectors_in_media = 0
     end

     rule_selectors_count = count_selectors_of_rule rule
     selectors_count += rule_selectors_count

     if rule =~ /\A\s*}\z$/
       current_media = nil
       # skip the line if the close bracket is the first rule for the new file
       next if first_hit
     end

     if selector_range.cover? selectors_count # add rule to current output if within selector_range
       if media_part
         output << media_part
       elsif first_hit && current_media
         output << current_media
       end
       selectors_in_media += rule_selectors_count if current_media.present?
       output << rule
       first_hit = false
     elsif selectors_count > selector_range.end # stop writing to output
       break
     end
   end

   if current_media.present? and selectors_in_media > 0
     output << '}'
   end

   output
 end

 # count selectors of one individual CSS rule
 def self.count_selectors_of_rule(rule)
   parts = strip_comments(rule).partition(/\{/)
   parts[1].empty? ? 0 : parts.first.scan(/,/).count.to_i + 1
 end



 # count selectors of a CSS stylesheet (not used by SprocketsEngine)
 def self.count_selectors(css_file)
   raise "file could not be found" unless File.exists? css_file

   rules = split_string_into_rules(File.read css_file)
   return if rules.first.nil?

   rules.sum{ |rule| count_selectors_of_rule(rule) }
 end
 
 
 
 private

   def self.extract_media(rule)
     if rule.sub!(/^\s*(@media[^{]*{)([^{}]*{[^}]*})$/) { $2 }
       $1
     end
   end

   # extracts potential charset declaration from the first rule
   def self.extract_charset(rule)
     if rule.include?('charset')
       rule.partition(/^\@charset[^;]+;/)[1,2]
     else
       [nil, rule]
     end
   end

   def self.strip_comments(s)
     s.gsub(/\/\*.*?\*\//m, "")
   end
 
end

# Set this to the root of your project when deployed:
http_path = "../"
css_dir = "css"
sass_dir = "scss"
images_dir = "images"
javascripts_dir = "js"
fonts_dir = "fonts"

# You can select your preferred output style here (can be overridden via the command line):
# output_style = :expanded or :nested or :compact or :compressed
output_style = :expanded

# To enable relative paths to assets via compass helper functions. Uncomment:
# relative_assets = true

# To disable debugging comments that display the original location of your selectors. Uncomment:
# line_comments = false
line_comments = true

# If you prefer the indented syntax, you might want to regenerate this
# project again passing --syntax sass, or you can uncomment this:
# preferred_syntax = :sass
# and then run:
# sass-convert -R --from scss --to sass sass scss && rm -rf sass && mv scss sass

on_stylesheet_saved do |path|
  Splitter.split(path) unless path[/\d+$/]
end