Take a some Shopify content, such as product.description or page.content and add `srcset` to any of its images.

_scan_string.liquid

{% capture func %}
  {% comment %}
  
    Scan String:
    Scan a string looking for things
    
    Parameters:
      Content: the content to be scaned
      Split: the item to split the string with
      Start: the thing that says we can start collecting
      Stop: the thing that says we can stop collecting
  
  {% endcomment %}
  
  {% assign parts = content | split: split | join:'~~' | replace: '>', '>~~' | replace: '<', '~~<' | split: '~~' %}
  {% assign capturing = false %}
  {% capture captured %}{% endcapture %}
  
  {% for a in parts %}
    {% unless a == blank %}
      {% if a == start %}
        {% assign capturing = true %}
      {% endif %}
      {% if capturing %}
        {% capture captured %}{{ captured }} {{ a }}{% endcapture %}
      {% endif %}
      {% if capturing and a contains stop %}
        {% assign capturing = false %}
      {% endif %}
    {% endunless %}
  {% endfor %}
  
  {% assign _scan_string_result = captured | replace: '>', '>~~' | replace: '<', '~~<' | split: '~~' %}
{% endcapture %}

_srcset.liquid

{% capture func %}
  {% comment %}
  
    SRCSET:
    Take a some content, such as product.description or page.content and add `srcset` to any of its images.
    
    Parameters:
      Content: the content to be searched
      Widths: a comma separated list of UNITLESS widths to be used in the srcset attribute
      Sizes: a string to be used as the sizes attribute value
  
  {% endcomment %}

  {% assign new_content = content %}
  {% assign ws = widths | split:',' %}
  {% include '_scan_string' content: content, split: ' ', start: '<img', stop: '>' %}
  {% assign imgs = _scan_string_result %}
  
  {% for i in imgs %}
    {% unless i == blank %}
      {% assign attrs = i | split: ' ' %}
      {% for a in attrs %}
        {% if a contains 'src=' %}
          {% assign src = a %}
          {% assign file = a | split: '/' | last | split: '?' | first %}
        {% endif %}
      {% endfor %}
        
      {% if file %}
        {% capture srcset %}{% endcapture %}
        {% for w in ws %}
          {% assign x = w | append: 'x' %}
          {% capture srcset %}{{ srcset }}{{ file | file_img_url: x, format: 'pjpg' }} {{ w }}w{% unless forloop.last %}, {% endunless %}{% endcapture %}
        {% endfor %}
        
        {% capture new_src %}src="{{ file | file_img_url: '64x', format: 'pjpg' }}" srcset="{{ srcset }}" sizes="{{ sizes }}"{% endcapture %}
        {% assign new_content = new_content | replace: src, new_src %}
      {% endif %}
    {% endunless %}
  {% endfor %}
{% endcapture %}{{ new_content }}

product.liquid

{% include '_srcset' content: product.description, widths: '320,640,768,900,1024,1200,1440,2048', sizes: '100vw' %}