I Just spent about half an hour trying to find out why chrome and firefox kept adding
display:none!important to an image I’d named adv.png. The behavior was absent in Edge.
I just learnt about Windows not allowing con as a filename, so i wasted time trying to find out if that could be the case here.
I wonder why I didn’t check extensions first. Turns out AdBlock Plus was just doing it’s job.
Thought it’d be wise to post this somewhere incase someone runs into a similar issue in future.