In brief: A 404 page appears when the server cannot find the requested URL, together with HTTP 404. It should explain the situation and offer a useful way forward.
How I use a 404 page in practice
I test it on real content and devices, not only in a design. I care about first impression, readability, information order, error states and the ability to go back. Observing a few people often reveals a more specific problem than a long team discussion.
What to watch
A beautiful output is not automatically usable. A 404 page can be attractive while hiding important information, slowing the page or making control harder. Do not test only on a large monitor with ideal content.
Questions for decisions
- What task does the person want to complete?
- Where might they hesitate, make a mistake or lose context?
- Does it work on mobile, with a keyboard and on a slow network?
- What signal will show that the change helped?