Ads or 3rd party content
I block third party content, because that's very effective against malware and saves bandwidth and speeds page load time.
Many adverts use 3rd party domains and images. Most obnoxiously they use scripts instead of simply being an image with a link.
When are websites going to stop their stupidity, esp, BBC*, CNN, all the main newspapers etc?
(*BBC is truly obnoxious if you do not have a UK IP address.)