Yeah, they work fine 90% of the time. But no web browser implements all webstandards and even less so implements them without bugs. Google is also adding more

Yeah, they work fine 90% of the time. But no web browser implements all webstandards and even less so implements them without bugs. Google is also adding more and more proprietary APIs to their browser, which Mozilla will not be able to implement (Chromecast, Google Earth, Hangouts) or is adding pseudo-webstandards, which have been publicly specified, but they did not push it through the standardization process to reach an agreement with other browser vendors (e.g. WebSQL, File Storage API). You're also saying "plenty of browsers". There's hardly any browsers that don't use Trident/EdgeHTML, Gecko or KHTML/WebKit/Blink. If they use one of those, then there's the major browsers IE/Edge, Firefox or Safari/Chrome that webpage owners will test/build against and as a result also cause it to work on that fork, meaning that yeah, those forks can easily implement an ad blocker without much to fear. But that's not the case, if you are such a major browser yourself. Then webpage owners will notice and will not have any reason to optimize against your browser engine anyways.