404 page or page not found pages or 404 error page, etc all are the pages that are used to display the error message when any link related to your website is not available or broken. These error pages play a vital role to interact with your audiences when your page is dead is link is broken. When a user tries to follow a broken or dead link, the website hosting server will often generate a "404 Not Found" web page; thus, the 404 error is one of the most well-known problems on the Internet.
It is very easy to make a 404 page not found error page using HTML and CSS. What you have to do is just create a static HTML page and write a 404 error message. In addition to this, you can provide links to other pages link home page, search pages, etc to redirect your visitor inside your website.
Following is a code to create a simple but responsive 404 error page using HTML and CSS. I recommend you to follow the code along with this video :
you can download the files from here.