According to a poll on Plinko, only 23% of visitors that encounter a 404 page make a second attempt to find the missing page. That means 77% of visitors that encounter a 404, leave your website, (probably never to return again). By creating a custom 404 page, you can increase visitor retention and help them find what they couldn't find the first time.
Website validation is one of the hidden secrets of effective web design. Even though validation has been around for a long time and readily available, it is under utilized. Part of the reason is that when you start to design your creative website, the last thing on your mind is: does my site validate? Somewhere along the way of designing graphics, layout and content, we said, "goodbye" to HTML standards and "hello" to whatever looks good in our web browser. So before we go any further, let's make sure we understand what validation is.