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

Okay, I think I have been missing an important piece.

it's just that assets:precompile task doesn't produce the secondary ie file.

That's right, the asset pipeline cannot produce extra files. It has to be there upfront.

Have a closer look at the CssSplitter::SprocketsEngine#evaluate:

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

All it does is evaluating / manipulating an existing asset file. The workaround we came up with is providing the same file multiple times with the same content but based on the file extension different parts are extracted using the CssSplitter.

So we have a ie8_portion2.css.split2:

//= include 'ie8.css'

See complete asset folder structure

The regex matches the 2 of the file extension split2 and hands it on to the CssSplitter: CssSplitter.split_string(data, part.to_i)

The result is a compiled ie8_portion2.css - including only the second part > 4095 selectors.

You could add further files if your CSS file is bigger than 8190 selectors.

This mechanism works fine on a Rails 3.1 project with asset pipeline turned on and fully automated asset compilation during deployment. Let me know if this does not work for you.

Hi @ChristianPeters,

pretty nifty workaround with the .splitx file endings.

Any plans to turn this into a gem?