One of my clients is an e-commerce website which ranks 6-10th for their keywords. Apart from the other SEO technniques, I suggested them to reveiw their code and optimize as per the HTML standards mentioned on this Github link: https://gist.github.com/ryansechrest/8693303
I did Google Lighthouse Audit, and score is almost 70+ for performance, accessibility and best practices. Although, I can see that their HTML coding is not upto the mark.
When I suggested them the above HTML coding practices, they said its gonna incurred a lot of cost.
They want to know how much SEO ranking will improve if we follow a certain coding standard.
Is there any guide, tips, documentation or suggestions available?