The 2nd paragraph of the "cycle" section (The HTTP request/response cycle) has an invalid link. See https://struts.apache.org/primer.html#cycle
This is the text - If you are not familiar with the HTTP request/response you should learn about it. There is a lot of help available in the internet, like How does the Internet work?
The link is to https://webplatform.github.io/docs/wiki/tutorials/How_does_the_Internet_work. that page is 404 not found and not coming back. https://webplatform.github.io/docs/ says "Notice: The WebPlatform project, supported by various stewards between 2012 and 2015, has been discontinued. This site is now available on github."
IMHO we can just remove that 2nd paragraph as it is not needed.