Whenever I try to navigate to the about.html page, be it by adding the ‘about’ in my URL manually in the browser, or by navigating to it through a link, it wants to get its assets from /about/assets/ rather than /assets/. Any links, ex. <a href="art.html">, also lead to /about/page (i.e. /about/art.html), which displays the correct HTML files, but again with the assets all broken…
The issue only happens locally. How can I fix this?
The URL in <a href="art.html"> is a Relative URL. I.e. it is relative to the current page, so when it is evaluated it will inherit the current page’s URL (as you’ve seen). Most of the time one should use Absolute URLs, in particular Domain-Implicit-Absolute-URLs. In this case, the link should be <a href="/art.html"> (note the leading slash, which makes the path absolute). More about URLs:
Jekyll has many facilities for managing a site’s URLs. Here’s a good article on the topic: