Add CSS check in didCutTheMustard method
/**
 * main.js
 *
 * We keep things simple here on the countryside. No complex
 * module loading or whatever using Browserify or RequireJS.
 * Just one plain, simple vanilla JavaScript file.
 */

/* global CSS */

'use strict'

// Only run in capable browsers via feature detection
var didCutTheMustard = 'querySelector' in document &&
  'classList' in document.createElement('a') &&
  'addEventListener' in window &&
  CSS.supports('display', 'flex')

if (didCutTheMustard) {
  // Add class "js" to html element
  document.querySelector('html').classList.add('js');

  /**
   * Add line numbers to <pre>
   */
  (function () {
    var pre = document.querySelectorAll('pre')
    var pl = pre.length

    for (var i = 0; i < pl; i++) {
      pre[i].insertAdjacentHTML('beforeend', '<span class="ln" aria-hidden="true"></span>')
      var num = pre[i].innerHTML.split(/\n/).length

      // Start counter at 1 to circumvent Jekyll adding an extra line in <code>
      for (var j = 1; j < num; j++) {
        var lineNum = pre[i].querySelector('.ln')
        lineNum.insertAdjacentHTML('beforeend', '<span></span>')
      }
    }
  })()
}

/**
 * main.js
 *
 * We keep things simple here on the countryside. No complex
 * module loading or whatever using Browserify or RequireJS.
 * Just one plain, simple vanilla JavaScript file.
 */

'use strict'

// Only run in capable browsers via feature detection
var didCutTheMustard = 'querySelector' in document &&
  'classList' in document.createElement('a') &&
  'addEventListener' in window

if (didCutTheMustard) {
  // Add class "js" to html element
  document.querySelector('html').classList.add('js');

  /**
   * Add line numbers to <pre>
   */
  (function () {
    var pre = document.querySelectorAll('pre')
    var pl = pre.length

    for (var i = 0; i < pl; i++) {
      pre[i].insertAdjacentHTML('beforeend', '<span class="ln" aria-hidden="true"></span>')
      var num = pre[i].innerHTML.split(/\n/).length

      // Start counter at 1 to circumvent Jekyll adding an extra line in <code>
      for (var j = 1; j < num; j++) {
        var lineNum = pre[i].querySelector('.ln')
        lineNum.insertAdjacentHTML('beforeend', '<span></span>')
      }
    }
  })()
}
