Coverts pixel values to em based on the font size set to the html tag.

pixels-to-ems.js

/**
 * Converts pixel values to em values based on the `html` level `font-size`.
 * @method pixelsToEms
 * @param  {Number|String}    pixels - pixel value to be converted to ems.
 * @return {Number}                  - value in ems.
 */
function pixelsToEms(pixels) {
  let html = document.querySelector('html');
  return pixels / parseFloat(getComputedStyle(html)['font-size']);
}