That's what we did, the first time. We carefully constructed a responsive design that would scale to an iPad resolution and make good use of the screen space, and that would scale again to a smartphone, taking into account single-handed use, UI guidelines for the size of controls so they could be tapped reliably on a high-res screen, and all that jazz.
And we learned in the process how many different ways Apple deliberately break the web browsers in their mobile devices.
They pretend they're on a different size of screen that has no relationship to physical reality, and scale your page accordingly. I'm sure that's great if you really want to see an entire web site squeezed into a few square inches with text too small to read, but it completely breaks the usual CSS tools for building a responsive design.
You can fix that by forcing the viewport size. However, if you do then as soon as the user rotates their iPad the browser zooms the page to show the same content at a different scale instead of actually letting the responsive design do its work by reflowing the layout to take advantage of the new page dimensions while keeping everything at the same size.
You can fix that by setting a maximum zoom level, and a lot of developers stop here, but if you do then you'll sacrifice accessibility for users who genuinely want to zoom because they have imperfect vision and have difficulty enjoying a site at a size that is aimed at a typical visitor.
You can fix that by setting the default zoom instead, detecting an orientation change event, and resetting your overrides each time the page rotates, as long as you can get all the proprietary hooks right and you don't mind either browser sniffing on your server or shoving the extra code to all your users even though most won't benefit.
And so on, and so on.
If Apple want usability for web sites on their iOS devices, they need to stop making it actively difficult for web developers to support their users. A single, simple option that says "make 1px = 1 physical pixel" would be sufficient to fix almost everything they get wrong as far as responsive design goes. Is that really so much to ask?
<cynic> But maybe they don't, because after all they take a hefty cut of the profits on anything you make via an app, so they have a vested interest in pretending to support web standards but actually screwing them up to make app development more attractive. </cynic>
If users don't like the experience they get as a result, there is an easy solution: don't buy Apple products that are shiny but broken, buy something else that works instead. There's no shortage of either good smartphones or good tablets from other sources today.
(Edit: FAOD, the "we" above refers to my team and me, developing our own products. We're not connected to the site linked at the top of this discussion.)