Making HTML Websites Mobile-Friendly

Good morning

I wonder if someone could help me. I'm in the process of building a website for a client and at the moment am creating it solely in HTML - it's a simple website so there's no need for anything more complicated.

My query is this: I'm an iPhone user and there's nothing that annoys me more than filling in webforms on my iPhone and not being presented with the correct keyboard for the task. For example, I'm being asked to enter a string of numbers but am given the default keyboard rather than the solely numeric one, or being asked to provide an email address but not having the @ symbol readily available.

I've seen some websites where tabbing into a numeric-only field gives you a numeric keypad to work with, and tabbing into an email address field makes the @ symbol appear on the main keyboard. Therefore, it must be possible to do it, but I'm not sure how. Ideally it would be good to be able to make it friendly towards all mobile devices (Windows, Android and Apple).

Any thoughts?

TIA

Ian