button.css

.test {}
.test:hover {
    color: blue
}
.test {}
.test:hover {
    color: blue
}
.test {}
.test:hover {
    color: blue;
}

button.ess

import * as merge from 'utils-merge'

function Constructor(opts)
  var opts = merge(opts, opts.defaults)
  var sel = opts.sel || ''

  `sel`
    if opts.children
      = opts.children

  each name, i in opts.blocks
    var pair = opts.blocks[i]
    var block = opts[pair[0]]
    if pair[0]
      `sel + '&' + pair[1]`
        if typeof block === 'object'
          each value, decl in block
            = decl + ': ' + value
        else
          = block

function Button(opts)
  var out = merge(opts, {blocks: [['hover',':hover']]})
  = Constructor(out)

.test
  Button(defaults={hover:{color:'blue'}})
  Button(hover={color:'blue'})
  Button
    block hover
      color blue