Don’t worry, as long as there aren’t too many errors and your blog is proven to be indexed by Google, then it can be said “Valid HTML has nothing to do with SEO & Google Index!”
The results of “online studies” aka Googling show, valid HTML & CSS templates have no effect on SEO and Google Index. Google still has the principle of Content is King and prioritizes blogs with Quality & Fresh Content!
So, don’t waste time dealing with valid HTML & CSS. Provided not too many errors, just enjoy! Google “don’t care”! If there are too many errors, the blog’s appearance will also be “abnormal”.
If it’s — data structure — if the error must be corrected, because that’s a picture of how our blog posts are indexed and appear on the Google search results page (SERP).
Again, don’t be “sad” and “pessimistic” if your blog template is not valid HTML & CSS, as long as it’s not too much and the blog looks normal.
On the other hand, for bloggers whose blog templates are valid HTML, don’t be too “PD” let alone “arrogant”. Valid HTML & CSS has nothing to do with SEO! (http://exampleblognih.blogspot.com).*