Can an ad extension add advertisements on a page visited by a user?
The fact that the checkbox is checked suggests its acceptability until you declare it, so users are aware of this.
If so, what is an acceptable policy?
I would say that everything that makes it clear to users what you are doing and follows the terms of the advertising network.
Can the extension replace existing advertisements (although it seems unethical / thieves to me) or should it only create new ones?
You must admit that this is unethical, most of the content and applications there cost money, and this deprives publishers. But, as is the case with a large number of extensions, it is considered by the browser as user-defined. Basically, how the website works - users control the client. Ad blockers are the most popular browser extensions, so I doubt the Chrome team will ban the extension that will change ads. Please think about website owners. Adding ads is at least better than replacing them.
Can I use any ad network or should it be adsense?
Anyone I'm sure. Google wants Chrome to be considered completely independent of its services. For this reason, you’ll even see Google various competitors promoted in the Chrome Web Store.
* However * , there is a big caveat here. It is possible that such an advertising insert is prohibited by the advertising network in question. This, of course, is the case with many affiliate links that you cannot just enter your own or change your own link. The argument is that the user will still click on it. Therefore, if you enter ads, the biggest limitation will be your advertising agent, not Chrome.
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