Adapted for Rails Asset Pipeline: Split CSS files so that there are no more than a given number of selectors in one style sheet. This is a tool to cope with Internet Explorer's limitation of max. 4095 selectors per stylesheet.

config-application.rb

#...

module MyProject
  class Application < Rails::Application
    config.assets.precompile += %w( ie6.css ie6_portion2.css ie7.css ie7_portion2.css ie8.css ie8_portion2.css ie9.css ie9_portion2.css)

    #...

config-initializers-sprockets.rb

require 'css_splitter'
Rails.application.assets.register_engine '.split2', CssSplitter::SprocketsEngine

ie8_portion2.css.split2

//= include 'ie8.css'

layouts-application.html.haml

    -#...  
  
    -# Rails Dev Boost compiles all templates that are included here regardless of browser requests
    -# FIXME: Consider to remove Rails Dev Boost and this switch after updating to Rails 3.2
    -# Switch testing_for to e.g. :ie7 when testing for ie7
    - testing_for = nil
    - if !Rails.env.development? || testing_for == :ie6
      /[if IE 6]
        = stylesheet_link_tag :ie6
        = stylesheet_link_tag :ie6_portion2
    - if !Rails.env.development? || testing_for == :ie7
      /[if IE 7]
        = stylesheet_link_tag :ie7
        = stylesheet_link_tag :ie7_portion2
    - if !Rails.env.development? || testing_for == :ie8
      /[if IE 8]
        = stylesheet_link_tag :ie8
        = stylesheet_link_tag :ie8_portion2
    - if !Rails.env.development? || [:ie6, :ie7, :ie8].include?(testing_for)
      /[if lt IE 9]
        = javascript_include_tag "//html5shim.googlecode.com/svn/trunk/html5.js"
        = javascript_include_tag 'ie/selectivizr'
    - if !Rails.env.development? || testing_for == :ie9
      /[if IE 9]
        = stylesheet_link_tag :ie9
        = stylesheet_link_tag :ie9_portion2

lib-css_splitter.rb

require 'tilt'
module CssSplitter

  class SprocketsEngine < Tilt::Template
    def self.engine_initialized?
      true
    end

    def prepare
    end

    def evaluate(scope, locals, &block)
      part = scope.pathname.extname =~ /(\d+)$/ && $1 || 0
      CssSplitter.split_string data, part.to_i
    end
  end

  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?

    file_id = 1 # The infile remains the first file
    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

  def self.split_string(css_string, part = 1, max_selectors = MAX_SELECTORS_DEFAULT)
    rules = split_string_into_rules(css_string)
    extract_part rules, part, max_selectors 
  end

  def self.split_string_into_rules(css_string)
    strip_comments(css_string).chomp.scan /[^}]*}/
  end

  def self.extract_part(rules, part = 1, max_selectors = MAX_SELECTORS_DEFAULT)
    return if rules.first.nil?

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

    output = charset_statement
    selectors_count = 0
    selector_range = max_selectors * (part - 1) + 1 .. max_selectors * part

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

      if selector_range.cover? selectors_count
        output << rule
      elsif selectors_count > selector_range.end
        break
      end
    end

    output
  end

  def self.count_selectors(css_file)
    raise "file could not be found" unless File.exists? css_file

    rules = IO.readlines(css_file, '}')
    return if rules.first.nil?

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

    rules.sum {|rule| count_selectors_of_rule(rule)}.tap do |result|
      puts File.basename(css_file) + " contains #{result} selectors."
    end
  end

  def self.count_selectors_of_rule(rule)
    strip_comments(rule).partition(/\{/).first.scan(/,/).count.to_i + 1
  end
  
  private
  def self.strip_comments(s)
    s.gsub(/\/\/.*$/, "").gsub(/\/\*.*?\*\//, "")
  end
  
end

Hi @ChristianPeters,

pretty nifty workaround with the .splitx file endings.

Any plans to turn this into a gem?