The easiest option is to open the W3C report and send it to the developer. You do not need to learn HTML when a contractor or website-builder support team maintains the site.

Message for the developer

Hello,
Please check the home page with the W3C Nu HTML Checker.
Repair Error messages and the agreed Warning messages, starting with the first structural error.
Fix the root cause in the template instead of editing every page where possible.
Validate again and explain any warning that remains.

Technical notes

  1. Back up files and the database.
  2. Validate the canonical HTTPS URL.
  3. Start with the first error; one unclosed structure can cause many later messages.
  4. Prioritise invalid nesting, unclosed elements, duplicate IDs, forms and interactive controls.
  5. Look for the cause in shared headers, footers, product cards and components.
  6. Validate again after each small group of changes.
  7. Visually test the page, forms, navigation and JavaScript; validation does not replace functional testing.

Some warnings are recommendations and may be retained for a documented reason. Informational notices are outside this service's repair price.