Search Friendly 404 Page Overview
A 404 error page tells users and search engines that the URL was not found. It's the default catchall when users or search engines request an incorrect or outdated URL. Unfortunately, the default error pages that come with web servers are not user-friendly.
A search-friendly 404 error page helps people continue through the site and helps search engines remove the old, incorrect URLs at the same time. This document explains what makes a great page for people and search engines.
Custom 404 Page Recommendations
Required Page Content Elements
The following elements are found in the great search-friendly custom 404 pages.
Branding: Consistent branding helps users to instantly recognize that they've reached the site instead of a generic error page. Generic error pages usually confuse users and force them to bounce off the site immediately.
Clear error message: Having a clear error message lets users know that the resource they were looking for doesn't exist.
Apology: This sets the right tone by being empathetic to the user. The apology usually appears directly before or after the error message.
Next action: The next action should be simple and obvious for users. Prominent design elements and CTAs should lead them through their journey through the site. The suggestions below are optional, as long as there is a clear next action.
Links: At a minimum, a custom 404 error page should link to common resources like the home page, help, forums, contact us, etc.
Navigation: A well-designed site navigation helps users to find their way through the site. This is especially true when they've reached a custom 404 error page. You may wish to add a simplified version of the navigation to avoid overwhelming people with choices. You might also consider including it in a clearer format within the body of the error page as a specific next step.
Search box: Regular users of the site frequently use the search box to locate something they can't find. It is a de facto standard to add it to the top right of the page, although you can also place it at the center of the page as one of the primary ways for users to continue their site journey.
Contact details: Depending on the situation, providing specific contact information such as a phone number, email, social media, or help chat is what a user needs most.
Dynamic suggestions: Some sites are able to supply some suggestions for related content based on the prior URL. The suggestions might be relevant sibling pages or potential parent pages.
Implementing 404 Pages
404 pages should always return a 404 HTTP status code. Then search engines will not accidentally index it. Otherwise, it results in a "soft 404" and creates low value content issues with search engines.
Configuring the 404 page to return a 404 status code can be done through your server.
Additional Resources
36 brilliantly designed 404 error pages (note: while these are beautifully designed, many of these pages could provide a few more practical options to continue their journey)