getStrokeOffsetForBBox = elem => {
  const sw = elem.getAttribute('stroke-width')
  return !isNaN(sw) && elem.getAttribute('stroke') !== 'none' ? sw / 2 : 0
}
@param {Element} elem
@returns {Float}
@todo This is problematic with large stroke-width and, for example, a single
horizontal line. The calculated BBox extends way beyond left and right sides.