```coffeescript
# csso   = require 'csso'
cleanCss = require 'clean-css'
Bundle   = require './bundle'
fs       = require 'fs'
os       = require 'os'
path     = require 'path'

class Css extends Bundle
  constructor: (@options) ->
    @fileExtension = '.css'
    super

  minify: (code) ->
    return code unless @options.minifyCss

    try
      # csso.justDoIt(code, false, false)
      # csso.parse(code, 'stylesheet', true)
      return cleanCss.process(code, {
        keepSpecialComments:0 # remove all
      });
    catch err
      filename = path.resolve(os.tmpDir(), 'tmpcss.css')
      fs.writeFileSync(filename, code)
      console.error("CSSO", err, "temporary file at: ", filename)
      process.exit()

  render: (namespace) ->
    style = ''
    for file in @files
      if file.namespace == namespace
        style += "<link href='#{file.url}' rel='stylesheet' type='text/css'/>"
    return style

  _convertFilename: (filename) ->
    splitted = filename.split('.')
    splitted.splice(0, splitted.length - 1).join('.') + '.css'

module.exports = Css
```

Resolve issue with incorrect url for remote CSS when using addCssUrl, and bundle option is false. This results from the file.url property being a boolean value instead of a string, and the result is the link tag is output as <link href='true' /> rather than with the correct url. Looks like this was already fixed for JS files, but not CSS.
user
```coffeescript
# csso   = require 'csso'
cleanCss = require 'clean-css'
Bundle   = require './bundle'
fs       = require 'fs'
os       = require 'os'
path     = require 'path'

class Css extends Bundle
  constructor: (@options) ->
    @fileExtension = '.css'
    super

  minify: (code) ->
    return code unless @options.minifyCss

    try
      # csso.justDoIt(code, false, false)
      # csso.parse(code, 'stylesheet', true)
      return cleanCss.process(code, {
        keepSpecialComments:0 # remove all
      });
    catch err
      filename = path.resolve(os.tmpDir(), 'tmpcss.css')
      fs.writeFileSync(filename, code)
      console.error("CSSO", err, "temporary file at: ", filename)
      process.exit()

  render: (namespace) ->
    style = ''
    for file in @files
      if file.namespace == namespace
        url = if typeof file.url == 'boolean' then file.file else file.url
        style += "<link href='#{url}' rel='stylesheet' type='text/css'/>"
    return style

  _convertFilename: (filename) ->
    splitted = filename.split('.')
    splitted.splice(0, splitted.length - 1).join('.') + '.css'

module.exports = Css
```
assistant