浏览器中没有活动标签时CSS不更新？

I'm using a progress bar for a timer in my game and I set it to go from 100% to 0% over 15 or so seconds. However, during testing since I have two open two tabs to simulate two clients I've noticed that the progress bar will only begin animating in each tab once that tab is activated for the first time.I'm using a progress bar for a timer in my game