*HTML* is the extension. I have yet to find anyone who doesn't know that all webpages end in htm or html. Its very common to get webpages with the extension. Almost any placwe you visit in the web will have the html part at the end.
But as Harry says, usually webservers are configured to load index.htm or index.html automatically without the need to type it, if it exists.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.