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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.
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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.
Below is an example of Seer's 404 page. This page clearly states that the page the user was looking for no longer exists and offers the user options to navigate away from the page quickly through the list of suggested links.
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)
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