An example of how to use srcset in Shopify themes

srcset.liquid

{% for product in collection.products %}
<a href="{{ product.url | within: collection }}">
    {% assign image = product.featured_image %}
    <img src="{{ image.src | img_url: '480x' }}" alt="{{ image.alt | escape }}"
        srcset="
        {% if image.width > 640 %}{{ image.src | img_url: '640x' }} 640w{% endif %}
        {% if image.width > 720 %},{{ image.src | img_url: '720x' }} 720w{% endif %}
        {% if image.width > 900 %},{{ image.src | img_url: '900x' }} 900w{% endif %} 
        " 
        sizes="(min-width: 960px) 450px, (min-width: 720px) 50vw, 100vw"
    />
</a>
{% endfor %}