All pages on a website are named pageone.htm, pagetwo.htm, pagethree.htm etc. but so that they look tidier on Googles pages, references to these pages on a website are like pageone, pagetwo, pagethree etc, with the relevant redirecting for this to auto happen.
The question is this - is the correct way, Google/Bing wise - SEO wise please?
I suppose it would come under the umbrella of “Keeping a simple URL structure”, though since Google’s primary concern is the relevancy of the content, I don’t believe it will have any huge impact.
But I think on the whole, hiding extensions is better. One way it can help is to future-proof the site. If you were to change from static html to dynamic server-side processed pages, the pages will change from about.html to about.asp or about.php.
By removing the extension from the indexed pages you won’t need to set up redirects and have to wait for Google to re-index your pages under their new names (which can cause a temporary ‘blip’) and any existing back links with remain current.