I wish to monitor the CSS of a few websites (these websites aren't my own) for changes and receive a notification of some sort when they do. If you could please share any experience you've had with this to point me in the right direction for how to code it I'd appreciate it greatly. 
I'd like this script/app to notify a Slack group on change, which I assume will require a webhook. 
Not asking for code, just any advice about particular APIs and other tools that may be of benefit.
I would suggest using Github in your workflow. That gives you a good idea of changes and ways to revert back to older versions.
How to monitor changes to a particular website CSS?