Web pages aren't designed to always be static and sometimes links will be removed.
This question refers to "P. 125 of these notes".
Unfortunately the link has been removed. Whilst in this case the question is still clear, it might not always be the case, and the link would add greater context to the question and might also provide further insight.
In order for a question to still be helpful and relevant, might it be worth Stack Exchange saving a copy of the links, perhaps similar to Google?