/etc/hosts entries blocking annoying game site (slither.io) in Chrome & Firefox, but not Safari
It's a simple solution. You don't need to mess with /etc/hosts file. Just use openDNS, sign-up, sign-in then goto the "Web Content Filtering" on the settings and tick the "Games" box.

Don't forget to change the DNS server on your computer to openDNS:

1) Unencrypted DNS: 208.67.222.222 and 208.67.220.220

2) Encrypted DNS: https://doh.opendns.com/dns-query

Check whether you have setup the DNS correctly through:

https://welcome.opendns.com

Good luck!