We had a weird graphical glitch on the timeline webpart on our SharePoint installation, we couldn’t figure out what was happening. Until that is we checked out the element in F12 Developer tools, within one of our customisations we used the .ms-rtestate-field img directive within our custom site CSS file.
So this is what we saw:
To fix, I commented out this directive from our main.css (i.e. our custom CSS file), this resolved the problem, it also didn’t appear to affect any of the other pages.