In the timeline if there is an event or era that has end date, the year in the end date always has these two lines following. See the image below. I took this screenshot in a timeline that had no CSS applied just to make sure it wasn't me.
Steps to Reproduce
Make an event or era in timelines
Give it a start year, and an end year
Look at it in world view mode, not edit
and hopefully you'll see two random lines after the end date year too