That's a good point. But I think there are some ways around that which aren't too intrusive. If you just render a fallback on the server side with a message ask

That's a good point. But I think there are some ways around that which aren't too intrusive. If you just render a fallback on the server side with a message asking users to support you in some way, and then replace that fallback with your ads, you can mark the ads as sponsored without having to mark the fallback. And if you have image based ads, you could mark the image itself using server-side processing to add some sort of watermark rather than a DOM-based marker.