To do this, you need to site root directory create a file named .htaccesscontaining code like this:
ErrorDocument 404 /404.html
After that, create a file in the root directory of the site 404.html
and write in it the code of the page that will be shown on 404 errors.
Similar settings can be made for errors 401, 403.
pagespeed
if included optimization settings... To fix the problem, disable optimization settings or not use links with similar keywords.To customize your 404 error page in popular CMS corresponding instructions are available: