Responsive images feature detection

ri-feature-detection.js

(function (window) {
      document.addEventListener("DOMContentLoaded", function (e) {
          var supports = {
            srcset: false,
            currentSrc: false,
            sizes: false,
            picture: false
          };
          var img = new Image();

          if ("srcset" in img) {
              supports.srcset = true;
          }

          if ("currentSrc" in img) {
              supports.currentSrc = true;
          }

          if ("sizes" in img) {
              supports.sizes = true;
          }

          if ("HTMLPictureElement" in window) {
              supports.picture = true;
          }

          console.log('Picture support: ' + supports.picture);
          console.log('sizes support: ' + supports.sizes);
          console.log('srcset support: ' + supports.srcset);
          console.log('currentSrc support: ' + supports.currentSrc);
      });
  }(this));

Would you be so kind to explain me why DOMContentLoaded event is necessary? If you run these checks before DOMContentLoaded, you will get different results?