An arbitrary long CSS selector splitter to go in your `config.rb` for Compass projects.

config.rb

# Split selectors that are too long so IE8 doesn't ignore them.
on_stylesheet_saved do |path|
    CssSelectorSplitter.split(path) unless path[/\d+$/]
end

# The following is a default config.rb created by `compass create`.

# Set this to the root of your project when deployed:
http_path = "/"
css_dir = "stylesheets"
sass_dir = "sass"
images_dir = "images"
javascripts_dir = "javascripts"

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

# 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


# 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

css-selector-splitter.rb

class CssSelectorSplitter
    # This has some horribly arbitrary code, documented below.
    # 
    # max_length: 1000~ seemed to work, but needs more testing to find
    #   actual breakpoint.
    #   
    # selector.each_slice(5): Determine greatest slice before exceeding
    #   max_length.

    def self.split(infile, outfile = infile, max_length = 1024)
        raise "infile could not be found" unless File.exists? infile

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

        output = nil
        new_file_string = ''

        rules.each do |rule|
            # Get the selector length.
            # @TODO: This can get the sections instead so we don't do so again
            # below, but that means running on every rule rather than just
            # those that are too long, and potentially increase runtime.
            selector_length = length_of_selector_in_rule rule
            if selector_length > max_length
                # If the length of this selector is larger than max_length,
                # split it up into smaller strings and give each the same
                # properties.
                puts 'splitting selector of length ' + selector_length.to_s
                new_rule = []
                # Split the rule into the selector and properties.
                sections = rule.split(/(\{[^}]*\})/)
                selector = sections[0].split(/,/)
                style = sections[1]
                # @TODO: Swap arbitrary each_slice position of 5 for greatest
                # position that doesn't exceed max_length characters.
                selector.each_slice(5).each_with_index do |small_selector, index|
                    small_selector = small_selector.join(',')
                    # Add a space to the end if not already there. Ruby returns
                    # the character code of a string index, not a new string
                    # like Javascript would.
                    if small_selector[-1] != 32
                        small_selector += ' '
                    end
                    # Add the style to this new, smaller selector, and put in
                    # our collection array to be joined into a string at the
                    # end of this loop.
                    # @TODO: Add system EOL.
                    new_rule[index] = small_selector + style
                end
                # Add the split rule to our string of infile.
                new_file_string += new_rule.join('')
            else
                # Otherwise add the rule as-is to the infile string.
                new_file_string += rule
            end

        end

        # Write out the new copy of infile. outfile is either infile or a passed
        # filepath, so infile is overwritten by default.
        output = File.new(outfile, "w")
        output.write new_file_string if new_file_string
        puts 'overwrite ' + outfile.to_s
    end

    def self.length_of_selector_in_rule(rule)
        # Split the rule on '{'. The first part is the selector string.
        return rule.partition('{')[0].length
    end

end

Please feel free to send me your updates on this to make it less arbitrary (read: less shite) and I'll more than happily merge them in =)

Thank you so much for your work on this! It is exactly what I was looking for.

A shame this Gist was so hard to find, took me well over an hour to stumble here (came from your Compass issue: Compass/compass#1193).