echo <input id=kmbefore><input id=kmafter onleave='alert( (kmafter.value - kmbefore.value) * priceperlitterperkm )'>

 > index.html to make it available to anyone, Worldwide, for free!

You are conveniently leaving out that you already must have:

* a server running 24/7 on the internet, paid for each month

* purchased, setup and keep paying every year a domain name

* configured a web server in that server, ideally with automated SSL certificate issue and renewal