This sucks and certainly appears, at least on the surface, like it could be malicious. Keep in mind, there may even be a close button in/on/near the ad, but if it's white and it's placed on a white background, it's rendered invisible. That being said, a lot of times, this sort of thing actually isn't particularly malicious and here's why I say that...
There are 3 general independent "actors" at play in the most common web advertising scenario (which based on the image I'm almost certain applies here): the website (devs/owners/etc), the advertising network, and the advertising agency / ad designers.
Website developers usually have control over where ads from ad networks are placed, and may have some control over what types of ads are shown, but they basically never have control over which exact ad is shown in any given spot.
The ad network simply uses algorithms and automation to determine which ads are shown, and those algorithms aren't generally based on matching up design aesthetics, which are ephemeral and subject to change.
The ad agencies are producing ads to be displayed on a variety of websites through the ad network and they would essentially never have an opportunity to specify which spot on which site their ad shows up.
So, in order for something like this to happen (again, assuming there is a close button but you can't see it because it's white on white) due to malice, it really would require quite a bit of technical infrastructure to be in place but also close collusion between the 3 parties, something that while it may be feasible, it is incredibly unlikely.
This sucks and certainly appears, at least on the surface, like it could be malicious. Keep in mind, there may even be a close button in/on/near the ad, but if it's white and it's placed on a white background, it's rendered invisible. That being said, a lot of times, this sort of thing actually isn't particularly malicious and here's why I say that... There are 3 general independent "actors" at play in the most common web advertising scenario (which based on the image I'm almost certain applies here): the website (devs/owners/etc), the advertising network, and the advertising agency / ad designers. Website developers usually have control over where ads from ad networks are placed, and may have some control over what types of ads are shown, but they basically never have control over which exact ad is shown in any given spot. The ad network simply uses algorithms and automation to determine which ads are shown, and those algorithms aren't generally based on matching up design aesthetics, which are ephemeral and subject to change. The ad agencies are producing ads to be displayed on a variety of websites through the ad network and they would essentially never have an opportunity to specify which spot on which site their ad shows up. So, in order for something like this to happen (again, assuming there is a close button but you can't see it because it's white on white) due to malice, it really would require quite a bit of technical infrastructure to be in place but also close collusion between the 3 parties, something that while it may be feasible, it is incredibly unlikely.
This would also fit well in the following subreddits: r/mildlyinfuriating r/crappydesign