My point is that you don't even need to mandate it for all sites, and attempting to do is kind of specious based on the existence of foreign sites. Rather you can focus on mandating it for the large consumer-oriented sites, and this will create enough of a critical mass that a web browser with parental controls enabled will have decent functionality.
The difficulty with forcing some uniform mandate onto "all sites" is that the mandate has to be for tags that are faithfully stated, rather than a blanket 18+. And small personal website operators shouldn't be in the position of being forced to determine whether the random stuff on their personal website is specifically suitable for 13+, 18+, etc.
That's the goal of defining the semantics in terms of an open system rather than a closed system - it fails gracefully.
> None of the current options are particularly good even if you are a parent that cares and is willing to invest time and effort.
Pragmatically this is disappointing to hear, but matches everything I've been able to surmise.
> The idea isn't for the site to communicate some arbitrary age appropriateness signal that they as a third party to the family couldn't possibly know. Rather it's to communicate classes of content such as porn, gambling, violence, social media, user generated content, games, that sort of thing.
I think it should be both. There should be a class of tags that assert a site is legally fine for a 13 year old to view in the US, an 8 year old to view in the US, etc, possibly multiplied with jurisdiction. (note the direction there - it's not a statement that there is content unsuitable for a 13 year old, rather it's a warranty that the contents are suitable for a 13 year old). There should also be tags of the content/aim of the site like you've listed.
The settings in the parental control software can then make a good first pass based on age, then content categories, then parents could even allow/disallow specific sites. The point is to provide good defaults, but ultimately keep control of parents rather than giving it away to corporate attorneys as any age verification (ie identity verification) based solution inherently does.