During theme development, is there a way to browse directly to a page?

For example, I need to work on the WebAuthN failure screen. Is there a path I can go directly to? It’s pretty time consuming to get there manually.

If you’re asking if you can go to an .ftl page directly by some path, no that isn’t possible. That’s because each .ftl page assumes that some specific set of attributes must be set in the scope before the page is rendered, so you can’t just load the page unless those attributes are set.