There could be lots of reasons for your issues, but the one that jumps out at me first is the old version of Safari that you are using. I understand that is associated with your OS X version, but… .
Many "modern web pages" assume your browser can handle some of the current programming techniques and features inherent in web sites.
Then again, it could be something completely different and simple.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.14.x,; iPhones, iPods and iPads galore!