The title tag is often what will be displayed in a link to the page in a search engine result page, so it is important to think in terms of the actually written content of the title tag. it also goes without saying that the title tag is a great resource to inform a user to something that is going on when it is a game from a complex front end system of some kind.
When using setTimeout as a way to go about updating the text that is displayed in a title tag, the time at which frames will run can change depending on the browser. For example if I set 33 milliseconds as the desired target time at which the next call to my loop will run, I often get a time around that much if the tag is active and I am not doing much of anything in the function call. However if I change tabs the time between calls will increase to 1000 milliseconds regardless of what value I set.
For example something like this:
Will update the title text to the amount of time between function calls, as expected it will give me around 33ms if the window is active, but will increase to a second if the window is inactive.