Does Googlebot care about valid HTML?
Why Webmasters Stack Exchange recommends it
Webmasters Stack Exchange answerers link it as an official Google statement that invalid HTML or CSS does not hurt indexing or SEO.
Recommended by 0 people across 0 different months, 2014 to 2017.
What it is
A short Google Webmaster Help video in which Matt Cutts answers a viewer's question about whether Googlebot cares if a page has valid HTML, noting that even google.com fails validation. He explains that the crawler is built to cope with messy markup and that validity is not a ranking factor.
What people said
Cites jeffatrackaid · 2014-07 · Webmasters Stack Exchange · on “Will a missing <head> tag prevent Google from indexing a WordPress site?”
Does Googlebot care about valid HTML? In this YouTube Video, Matt Cutts says [this video] that the crawler is built to deal with HTML syntax errors. Google does not penalize you if you have invalid HTML. Google places more emphasis on quality content not HTML syntax. So I do not expect a faulty <head> tag to be the only reason for the site not being included. Given this error, perhaps there are other errors that may be blocking the bot, such as your robots.txt file, rewrite rules or browser detection code. I would check the servers logs to see if Google is in fact visiting your site. Then check your markup and use Fetch as GoogleBot to test the site.
Cites Goyllo · 2016-04 · Webmasters Stack Exchange · on “Is using the <u> tag invalid or bad for SEO?”
Using <u> tag in any webpage does not affect in SEO. If I was there then I will check How Google view my page or render my page, you can check that on Google search console. Here is two [LINK] videos [this video] from matt cutts about HTML validation.
Cites TopQnA · 2017-09 · Webmasters Stack Exchange · on “Should I ignore W3 CSS Validation Service?”
I would say it's good to have valid scripts. Having said that when you use third-party products it may be hard make it a valid one. If you have a custom-designed solution then you can easily fix and make it fully valid. So, i would say don't worry too much but if it is within your capacity to make it valid then go ahead and make one. Modified: As stated, it's good to have valid HTML and CSS as it makes it easier to manage. But if you are using a third-party system and if it is too hard to make it a valid one the do not worry as search engine reads incorrect markup fine. Watch This: [this video]
Cites TopQnA · 2017-09 · Webmasters Stack Exchange · on “Should I ignore W3 CSS Validation Service?”
When i say be real. Look around and understand the reality. Again repeating again, its good to have valid makup but in many situations its not worth trying unless you designed it. So, i am fully aware what i am recommending. So, i am in support of valid markup but at the same time be real. What this: [this video]