A lot of our site is built around YUI library. A quote from their announcement back in 2014
Which basically means that none of the javascript modules are updated or patched any longer. So it's my understanding that since the code injected into the site from the YUI libraries is not kept up to date, it doesn't play ball nicely with newer browser updates and causes problems.
I'm not an expert at this stuff. Take what I say with a grain of salt. I could be wrong :)