application.html.haml

-# put this in your layout
-# what this does it is loads the split_asset application.css file using the correct sprockets name
/[if lt IE 10]                                                                                                      
  = stylesheet_link_tag stylesheet_path('application').gsub(/\/assets\//, '/split_assets/')

application.rb

# put this line of code into application.rb
config.middleware.use('StyleSplit', '/split_assets')

style_split.rb

# Put this file somewhere in your rails loadpath. I dropped it into lib.
# 
# based on code outlined at: http://bindle.me/blog/index.php/200/splitting-the-asset-destroying-arcane-ie-bugs-on-the-rails-rack
# and also based on: https://gist.github.com/1131536
#
class StyleSplit
  def initialize(app, path)
    @app = app
    @path = path
  end

  def call(env)
    if env['PATH_INFO'] =~ Regexp.new('^' + @path + '/([^/]*).css')
      filename = File.join(::Rails.public_path, ::Rails.application.config.assets.prefix, $1 + '.css')
      css = StyleSplit.split_css(filename)

      [200, {
        'Content-Type' => 'text/css',
        'Content-Size' => css.bytesize.to_s,
        'Cache-Control' => 'public, max-age=31536000'
      }, [css]]
    else
      @app.call(env)
    end
  end

  def self.split_css(infile, max_selectors = 4095)
    charset_statement, rules = self.read_rules(infile)
    return if rules.nil?

    rulesets = self.partition_rules(rules, max_selectors)

    charset_statement + rulesets[1].join('')
  end

  def self.read_rules(infile)
    raise "file could not be found: #{infile}" unless File.exists? infile

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

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

    return [charset_statement, rules]
  end
  # return an array of rulesets, each of which contains no more than max_selectors
  def self.partition_rules(rules, max_selectors)
    selectors_count = 0
    rulesets = []
    ruleset = []

    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
        # finish ruleset
        rulesets << ruleset

        # Prepare next ruleset
        ruleset = []

        # Reset count with current rule count
        selectors_count = rule_selectors_count
      end

      ruleset << rule
    end
    rulesets << ruleset unless ruleset.empty? # final ruleset

    rulesets
  end
  def self.count_selectors_of_rule(rule)
    rule.partition(/\{/).first.scan(/,/).count.to_i + 1
  end
end