How to check If an element is present in a webpage

I wanted to ask if anyone knew how to make a program which would constantly check a webpage until a specific element shows up which would then prompt an alert. For example it could check my youtube subscriptions until a video shows up called "Hello World" and then the program would inform me of this. FYI this is not what I will be doing - just an example. I am an avid python programmer but would find it hard to see how to do this, therefore I feel like c# would be needed for this. So if anyone knew how to help start this off for me I would be hugely appreciative. Also I would try to do this whilst running in the background so not using selenium etc and maybe use methods like HttpWebRequests.

Thanks again!