old_contents: import postcss from 'postcss'
  import { readFileSync as slurp } from 'fs'
  import { resolve, isAbsolute } from 'path'
  import { parse as parseUrl } from 'url'
  import { default as rebaseUrl } from 'postcss-url'
  
  export default function rebase(pkg, opts, filename) {
    return postcss([
        rebaseUrl({
          url: (url, decl, from, dirname, to, options, result) => {
            let parsed = parseUrl(url, true, true)
            
            if (parsed.host || isAbsolute(parsed.path)) {
              return url
            } else {
              let resolved = pkg.relative(resolve(pkg.root, opts.lib, parsed.path))
              return resolved
            }
          }
        })
      ]).process(slurp(filename, 'utf8'))
  }
new_contents: import postcss from 'postcss'
  import { readFileSync as slurp } from 'fs'
  import { resolve, isAbsolute } from 'path'
  import { parse as parseUrl } from 'url'
  import { default as rebaseUrl } from 'postcss-url'
  
  export default function rebase(pkg, opts, filename) {
    return postcss([
        rebaseUrl({
          url: (url, decl, from, dirname, to, options, result) => {
            let parsed = parseUrl(url, true, true)
            
            if (parsed.host || isAbsolute(parsed.path)) {
              return url
            } else {
              let resolved = pkg.relative(resolve(pkg.root, opts.lib, parsed.path))
              return resolved.replace(/\\/g, '/')
            }
          }
        })
      ]).process(slurp(filename, 'utf8'))
  }
subject: Replace `\` with `/` in resolved path