How can I know the IMEI of the mobile,
via an HTML site
Without using PhoneGap?
In a phone, only native app can have access to the IMEI number, not web apps. So the browser may have access if it has enough permissions but this information is not provided to the page. This is intentional because many things can be achieved by the site owner like tracking and for ads.
So, if you really want to have the IMEI, you would need to create a native app (like PhoneGap which you have said no to) and get permissions. If your intention is just to track the user in your site, you can assign a unique id for him/her in a cookie.