convertDocument(doc) { // eslint-disable-line
    throw new Error('This method is abstract')
  }
@param {Document}
@returns {DOMElement|DOMElement[]} The exported document as DOM or an array of elements
         if exported as partial, which depends on the actual implementation
         of `this.convertDocument()`.

@abstract
@example

convertDocument(doc) {
  let container = doc.get('body')
  let elements = this.convertContainer(container)
  let out = elements.map(el => {
    return el.outerHTML
  })
  return out.join('')
}