Looks like the feature is coming soon, maybe to v0.104 404 pages may soon start working on Hugo localhost tests!!! (previously, you had to explicitly navigate to localhost:1313/404.html)

Hugo server now supports 404 as of version 103

Part 2: Why do custom 404 pages not work on Hugo localhost?

Darthagnon September 15, 2022, 8:03pm 2

This is a continuation of my previous post on the subject, which has been closed.

Topic		Replies	Views
Why do custom 404 pages not work on Hugo localhost? support	2	1248	June 24, 2022
How to test 404.html on local server? support	2	9547	February 2, 2015
Add `hugo server` support for its own 404.html? feature	2	975	March 31, 2016
Customize 404.html when using hugo server	7	2143	August 10, 2017
404 when deploying to gh-pages support	5	370	August 26, 2022