Saving web page as an html file on computer

I want to save the source code for my website page into my computer. I know that I have to use an http request to download the source code for my web page into the computer as a html file. I want to run a diff to track changes between two html files for a web page. I am wondering how to implement a program to perform the function of saving a web page as an html file on my computer. Please help it is really appreciated.