Ah I was proposing the tag as another option - maybe a news article is fine for kids but the comment section is unmoderated. That way they could still read the article but not the comments

Better idea is just tell the server if you want to hide restricted content. Then it can make any changes necessary. Yes it's a fingerprinting bit. Not a new one, because they could already detect whether the restricted tag was rendering or not, anyway.

But a HTML document could contain a listing of things, that might or might not fall in to these categorize (a search result). A better option would be to just have it as an attribute to elements, like the "lang=..." attribute. That way an UA could even have a policy of show text tagged, but not images. Or images only when clicked per images. Websites wouldn't need to implement spoilers or blurs then, this could be implemented in the UA.