Paul, the short answer is no.

html is the standard name of a webpage, without filetype html your pages cannot be read by any webbrowser

You may also see .php used by Website X5

@Paul ... you mean maybe the extension .html ?

if yes read this:

https://alexcican.com/post/how-to-remove-php-html-htm-extensions-with-htaccess/

Act with caution. In case of error on the .htaccess file, the website may no longer be displayed...

...so first make a copy of the file

The X5 is built up in an old fashioned way! Today, no one is using the html tag. I have been waited a long time for the X5 to end this.