A few years back in about 2010 I ran the W3C.org validity checker tool on a Drupal 6 website that is no longer exist and I clearly remember getting a message like "usage of <u> tags isn't valid" or a similar phrase about <u> tags.
Did someone here ever heard of any recommendation not to use these tags? I find no reason whatsoever as to why this would be marked "invalid".
The reason it was marked invalid is because it was deprecated, i.e. at the time you tested it was no longer part of the current HTML spec.
It's now been reintroduced to the HTML5 spec, albeit with a different use. Helpful discussion here.
While there is a relationship, technical validity and desirability or efficacy for the purposes of SEO are not necessarily equivalent. You'll see plenty of technically invalid code performing perfectly well for SEO, perfectly valid code making no difference to SEO, and so on.
In short, provided you're using the <u> element as specified, it may or may not have SEO advantages but certainly won't be an SEO problem.
The <u> tag is used to designate a section of text that should be underlined. It is useful in instances where you want to indicate a misspelled word or a proper name in Chinese. I think it's not bad for SEO because search engines want you to indicate or underline some important words.
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.
It stays free to you to use or don't use this tag. It has no defined influence on SEO, and, in general, on any kind of web semantic. Even if the element was redefined in HTML5 after deprecating it in HTML4, HTML5 reminds developers, that there are almost always more appropriate elements than the <u>.
Beside of this all, using of <u> is highly deceptive in terms of UX, because all human experience with the web browsers and HTML pages shouts: underline equals link! Even if it isn't blue - it could be just already visited. So if one tries to click on an <u>word</u> and nothing happens - guess, what would one think about you as web designer? Well at this point we could say, yes: the <u>-tag negatively influences a page in terms of SEO, because it fools a user, who doesn't want to be fooled and leaves the page.
SEO mostly depends upon content extracted from web page, content analysis ,sentiment analysis etc.
List and Table li, are mainly data-block ,so,they are important.
tag does not have any significant effect on SEO. This is used for visual presentation of content.
To make web-content information, use suitable schema tags to pass useful data.
represent text that should be stylistically different from normal text, such as misspelled words or proper nouns in Chinese, so would likely result in a warning or note to use something else more appropriate. I'd suggest checking the W3C Validation Service instead.