The responsible thing indeed would be to place a marker onto the content. In the case of DeviantArt, there's a barely visible text placed underneath the description together with EXIF-data - which, in case you use their own imagegen, is paired with a ad for their own service:
![deviantart screenshot: "created using AI tools" barely visible, big "deviantart dreamup" ad](https://i.sstatic.net/pR2et.png)
In the case of dA it's quite interesting - the site is absolutely unrecognizable to the place of teens sharing art of questionable quality and subject it used to be known as. It now acts as a imagegen subscription with an attached sharing platform. They announced a policy of requiring AI tags on AI generated content and of course also made an AI to detect AI.
Given that they can monetize AI, deviantart probably has a quite decent incentive to mark things to the best of their ability. Most other websites lack this incentive.
I also have seen several articles on help centers and knowledge bases of the likes of Google and Microsoft with a banner saying something to the effect of "this material has been machine translated", though of course in recent months it's turned to "AI translated". Here's an example from Citrix which liked the idea so much they did it twice (yellow and blue banner) and a disclaimer paragraph ("Haftungsausschluss") at the bottom:
![citrix double banner warning me this page is machine translated](https://i.sstatic.net/1989R.png)
In practice, this banner is understood as "user beware, this page is probably somewhat low-quality, go check the original if you can speak English". This is fine for a Microsoft KB which has an interest in keeping trust with its business users, but absolute poison for the kind of SEO spam that Google and Bing are struggling with lately.[1] I therefore suspect that, short of some quite heavy regulation, we won't be seeing any sort of unified icon for it or similar server-side.
% estimate of AI generated content, or even that something is 100% human generated (and what that means).
I think there's a massive difference from the likes of "I used grammarly to improve my sentence structure" and "I used an imagegen instead of a stock image for the cover image" to "hey chatgpt write me a review about X following all the SEO rules there are".
A "how much of this content is AI-generated" appears similarly hopeless and useless as a score that measures how much of a website is UX designed: It's a bit of a weird metric to begin with and doesn't capture what I actually care about. In the case of SEO and AI, I'd want to know if the article is genuine and/or truthful, in the case of UX, I'd want to know if the UX is actually any good or if it's full of dark patterns to annoy me into doing something.