Now that the screenshots are available, they need to be labeled. The labeling phase operates on couples of comparable screenshots.
For example, improper loading of images, missing text, different design, different languages, different selections, missing bullets or checkboxes and others are marked incompatible.
Since the screenshots are taken at different times in the two browsers, there are differences which are not incompatibilities but are actually due to the different timing. For example, a banner could be showing a different advertisement, a video could be in two different frames, a clock could be showing different time, a captcha could be showing different characters or images, two news could be different, and so on.
In the training phase, the best case is that we are able to detect between Y + D and N. If we are not able to do that, we should at least aim for the relaxed problem of detecting between Y and D + N. This is why we have this three labeling system.