* Provide some example feeds in the left side bar when the list is empty, so people new to RSS aren't just looking at an empty, black page. Maybe https://news.ycombinator.com/rss ?
* Provide an OPML import option so experienced RSS-using users can quickly populate their list of feeds and compare your project with whatever they are currently using.
* Don't build HTML by interpolating arbitrary strings whose content you don't know 100% in advance. Actually, avoid interpolating at all if you can use a builder that takes care of all the escaping for you.
* Double-check that those `/getFeed?url=` and `/fetchArticleText?url=` (and any other similar stuff) don't recursively fetch URLs pointing to themselves or to the same domain they're being hosted at. And then triple-check again.