Now for the last 5 years or so this sort of effect has been achievable using standard style sheet css. However, unless someone bothers to remove the old stuff it simply sits there.
Why fix code if it isn’t broken?
Simply put, the result will be quicker load times. This pleases people because they don’t have to wait around for a site to load and pleases search engines because they look at load times when assigning quality scores to pages.
So, do yourself a favour and if your website was built more than 5 years ago, get someone to check it for old bloated code that is out of date.