lazy-loader.js

/*
  ```html

  <img data-src="/path/to/image.jpg" alt="">

  ```

  ```css

  img {
    opacity: 1;
    transition: opacity 0.3s;
  }

  img[data-src] {
    opacity: 0;
  }
  ```
*/
[].forEach.call(document.querySelectorAll('img[data-src]'), function(img) {
	img.setAttribute('src', img.getAttribute('data-src'));
	img.onload = function() {
		img.removeAttribute('data-src');
	};
});