How to optimize website for touch devices

It sounds to me like you want to have a touch-screen-friendly option, to cover the following scenarios:

1. iPhone-like devices: small screen, touch only
2. Small screens, no touch (you didn't mention this one)
3. Large screens, no touch (i.e. conventional computers)
4. Touch-screen-enabled large screens such as iPad, notebooks/pcs with touch screens.

For case 1 and 2 you will probably need a separate site or a CSS file that eliminates lots of unnecessary content and makes things larger and easier to read/navigate. If you care about case #2 then as long as the links/buttons on the page are keyboard-navigable then case 1 and 2 are equivalent.

For case 3 you have your normal website. For case 4 it sounds like you want clickable things to be bigger or easier to touch. If it's not possible to simply make everything bigger for all users, an alternate style-sheet can provide you with the touch-friendly layout changes.

The easiest thing to do is provide a link to the touch-screen-version of the site somewhere on the page. For well-known touch devices such as iPad you can sniff the user agent and set the touch stylesheet as the default. However I'd consider making this the default for everyone; if your design looks good on the iPad it should look acceptably good on any notebook. Your mouse users with less-than-stellar clicking skills will be pleased to find bigger click targets, especially if you add appropriate :hover or mouseover effects to let users know that things are clickable.

I know you said you don't want to sniff user-agents. But I'd contend that at this time the state of browser support for this is in too much flux to worry about the "Correct" way to do it. Browsers will eventually provide the information that you need, but you will probably find that it will be years before this information is ubiquitous.

Good news! The editor draft of CSS4 Media Queries have included a new media feature 'pointer'.

Typical examples of a ‘fine’ pointing system are a mouse, a track-pad or a stylus-based touch screen. Finger-based touch screens would qualify as ‘coarse’.

/* Make radio buttons and check boxes larger if we
   have an inaccurate pointing device */
@media (pointer:coarse) {
    input[type="checkbox"], input[type="radio"] {
        min-width:30px;
        min-height:40px;
        background:transparent;
    }
}

It's also possible to test the media query from JavaScript:

var isCoarsePointer = (window.matchMedia &&
                       matchMedia("(pointer: coarse)").matches);

Updated Feb. 11th. 2013 On Windows 8 recent versions of Chrome (version 24+) detect touch-hardware when launching the application and expose touch events. Unfortunately if "pointer:coarse" returns false, there is no way to know if it's because pointer media queries are not implemented or because there is a fine pointer. WebKit haven't implemented "pointer:fine" yet, so we can't check that either.

Update Sept. 26th. 2012 Tested in Safari on iOS6 and Chrome on Android 4.1.1 and it's not there yet. 'pointer' and 'hover' media-queries landed in WebKit May 30th. According to the User-Agent, Safari uses WebKit branch 536.26 from April 25th, and Chrome on Android uses and even older one (535.19). Not sure WebKit branches from User-Agent strings are to be trusted, but my test page is not able to detect pointer media queries either. The implementation from May only implements the pointer media query for touch devices, so pointer: fine won't work for devices with a mouse.

I don't know if a standardized media query like Mozilla's will solve the problem by itself. Like one of the Chromium developers said in that discussion you linked, the presence of touch event support in the browser doesn't mean touch events can or will fire, or even if they do, that the user will only want to interact via touch input. Likewise, the presence of touch input support in the device doesn't mean the user will use that method of input - perhaps the device supports mouse, keyboard, and touch input and the user prefers the mouse or some combination of the three input types.

I agree with the Chromium developer that supporting touch events was not a bug in the browser. A good browser should support touch events because it might be installed on a device that supports touch input. It's the website developer's fault that he took the event support to mean the user would be interacting via touch.

It seems we need to know two things: (1) What are all the supported input types on the device (2) What are all the supported event types in the browser

Since we don't know #1 right now, there is one approach proposed by PPK of quirksmode that I like. He talks about it here: http://www.quirksmode.org/blog/archives/2010/02/do_we_need_touc.html#link4

Basically, listen for touch events and mouse events, and when they happen, set up the UI accordingly. Obviously that's limiting to the developer. I don't think it's a valid approach to your problem with link size because you don't want to wait for interaction to alter the UI. The whole point is to present a different UI (a larger/smaller link) before any interaction occurs.

I hope you make your proposal and it gets included in CSS3. Until then, as much as it pains me to say it, user agent sniffing looks like the best approach.

p.s. I hope hope hope someone comes here and proves me wrong