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.
One possible solution:
Crawl website for .css files, save change dates and/or filesize. 
After each crawl compare information and if changes detected use slack API to notify. I haven't worked with slack that, for this part of the solution maybe someone else can give advice.
How to monitor changes to a particular website CSS?