<figure>
  <figcaption><code>sizes="320px"</code></figcaption>
  <img srcset="https://placehold.it/320 320w" sizes="320px">
</figure>

<hr>

<figure>
  <figcaption><code>sizes="100px"</code></figcaption>
  <img srcset="https://placehold.it/320 320w" sizes="100px">
</figure>

<hr>

<figure>
  <figcaption><code>sizes="20vw"</code></figcaption>
  <img srcset="https://placehold.it/320 320w" sizes="20vw">
</figure>

<hr>

<figure>
  <figcaption>No <code>sizes</code> attribute (browsers will assume <code>sizes="100vw"</code>)</figcaption>
  <img srcset="https://placehold.it/320 320w">
</figure>
figure {
  margin: 0 auto;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.