How to do lazy loading image with srcset?

Usually, to implement lazy loading in HTML, instead of src or srcset attributes, we use data-src or data-srcset so that browser does not load images during speculative parsing. Later on, when Javascript is executed, and the user has scrolled near the image element, we load the actual image and update the src or srcset attribute’s value.

Two very popular lazy loading libraries lazysizes and vanilla-lazyload support responsive images out of the box.

Here are a few examples of using lazysizes.

Lazy loading responsive images in srcset and sizes

<img
    sizes="(min-width: 1000px) 930px, 90vw"
    data-srcset="small.jpg 500w,
                 medium.jpg 640w,
                 big.jpg 1024w"
    data-src="medium.jpg"
    class="lazyload" />

Using low quality placeholder in lazy loading

<img
    src="low-quaity-placeholder.jpg"
    sizes="(min-width: 1000px) 930px, 90vw"
    data-srcset="small.jpg 500w,
                 medium.jpg 640w,
                 big.jpg 1024w"
    data-src="medium.jpg"
    class="lazyload" />

Lazy loading images in picture element

<picture>
      <source
          data-srcset="500.jpg"
          media="(max-width: 500px)" />
      <source
          data-srcset="1024.jpg"
          media="(max-width: 1024px)" />
      <source
          data-srcset="1200.jpg" />
      <img src="fallback-image.jpg"
          data-src="1024.jpg"
          class="lazyload"
          alt="image with artdirection" />
</picture>

You can learn more about responsive images from this guide - https://imagekit.io/responsive-images

I recommend responsivelyLazy. The implementation is SEO-friendly and does not mess your HTML code. Here is a snippet:

<div class="responsively-lazy" style="padding-bottom:68.44%;">
<img
    alt=""
    src="images/2500.jpg"
    srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
    data-srcset="images/400.jpg 400w, images/600.jpg 600w, images/800.jpg 800w, images/1000.jpg 1000w, images/1500.jpg 1500w, images/2000.jpg 2000w"
/>

As you can see the value in the src attribute is not modified.

Read more at http://ivopetkov.com/b/lazy-load-responsive-images/

lazySizes is just working fine. You need to alter your markup into something like this however.

<img data-src="better_me.jpg" data-srcset="better_me2.jpg 400w, better_me.jpg 200w" class="avatar photo avatar-200 lazyload" data-sizes="auto" alt="better_me" width="200" height="200" />

Note srcset is changed to data-srcset and data-lazy is changed to data-src. Additionally you must add the class lazyload.

Your sizes attribute didn't made too much sense. Maybe you want to use x descriptors instead? Or simply use sizes="200px"? I don't know. I simply switched it to data-sizes="auto", so it gets automatically calculated for you. (But in that case the image dimension has to be computable before the image is loaded.)

lazySizes indeed loads images before they get in view. This is a big improvement for user experience. A user, who scrolls something into view doesn't want to wait then. A lazyloader that starts downloading an image after it is already in view disrupts the user experience.

One nice thing about lazySizes is that this lazy loader checks whether the browser is currently heavily downloading and decides on this fact, whether it only downloads in view images or to also preload near view images.

But if you don't want this you can control this by setting the lazySizes' expand and expFactor options.