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Is there a way to make page layout fit perfectly at 100% zoom but also respect greater zoom factors?

There are several ways to layout web pages so they fit the browser viewport (DOM window). When I say "fit" I include changing the size of the fonts used and changing the width and perhaps heights of DIVs, IMGs, and other rendered elements, as well as allowing "fluid" movements of elements. The goal is to make the page look "nice" on all display devices, from tiny phones to big desktop screens. These layout ways include using CSS em and % sizes, wrapping of text and elements, and JavaScript run by the onload and onresize events. Layout fitting can be made to work almost perfectly, cross-browser, but a sophisticated result may be insensitive to browser zooming, since zooming changes the window width magically, which means without any browser-independent, reliable way to detect the zoom (DOM and CSS standards ignore browser zooming). There is no way to detect the difference between a small window width due to a small device screen versus a small window width due to a large zoom factor. We want to handle these two cases differently, since vision-impaired users need the zoom factor to be honored.
Has anyone succeeded in fitting layout really well (including changing font sizes) in a way that respects zooming? For example, if the user visits the page with a zoom of 200%, initially only the upper left quarter of the page should be shown (the rest being available by scrolling horizontally and vertically), while the same page would fit into the window perfectly for another user having their zoom set to 100%.

Indeed to have a fluid layout you will have to use % to determine the width instead of pixels.
And for fitting the screen use 100%

Just add the following metatag
<meta name="viewport" content="width=device-width,
initial-scale=1.0,
maximum-scale=1.0,
user-scalable=0">

Why javascript report 800px width for Motorola Atrix MB860 4G ? Shouldn't it be 540px

For my Mobile Web App I am trying to get screen width of the device to fit the views dynamically.
I am really confused why Motorola Atrix Atrix MB860 is reporting a 800px width. When I see the spec of this device its resolution is 540x960. Why in javascript it reports 800px? I used this test page:
http://www.quirksmode.org/m/tests/widthtest.html
I tested the above page on several devices and they all report width correctly apart from Motorola atrix. For instance galaxy s2 reports 480px and nexus 7 reports 1280px

It's an odd behaviour of Android 2.3. which i found out here: http://tripleodeon.com/2011/12/first-understand-your-screen
Android v2.3, in a regular browser scenario, however, displays even more curious behavior. Its screen.width always starts off as 800 – which is clearly some sort of virtual viewport, rather than the physical screen. But when the document has an XHTML-MP doctype, or a constrained viewport (for any doctype), the value will switch, by the time of the document load event, to be the 320 or 480 you might expect.
window.outerWidth should do the trick or set viewport width to device-width
<meta name="viewport" content="width=device-width, user-scalable=no" />

Redirect to mobile page when width is too small

Since my website doesn't look good at all on a small screen, I want to create a JS function that redirects me to a mobile version of the page when width of the screen is smaller than or equal to 800px.
Here is the code for it:
if (screen.width <= 800) {
document.location ="index_mobile.html";
};
If the code works, then when I shrink down the browser window to 800px wide, the index_mobile.html should show up. But it is not showing up right now. Does anyone know what's going on?
http://jsfiddle.net/RZMmV/

Mobile browsers do not report or use the real device resolution because this would make basically all websites on the internet unusable.
What they do is creating a "virtual screen" that has a resolution that is closer to the resolution of a desktop PC and then will implement zooming on the page.
If you want to know the real device resolution you need to disable automatic scaling done on the device. For example for iOS and Android devices this can be done adding
<meta name="viewport"
content="width=device-width; initial-scale=1.0; maximum-scale=1.0; user-scalable=0;"/>
to the <head> section of your page. This informs the browser that the page has been designed for handling low-resolution devices and disables the virtual screen layer.

screen.width will return the width of the monitor, not the window, so you can't just shrink your browser window down to get a different value. To do that, you'll want to use window.outerWidth or something.

Android browser setting element size too small

I have a website using the meta tag <meta name="viewport" content="width=device-width, target-densityDpi=device-dpi">, Javascript that formats the css for body to screen width and height, and elements inside that have percentage-based widths.
On desktop, this works fine. On my Android phone and tablet's browsers, though, the content isn't scaling properly - everything is just a little too small, and double-tapping to zoom zooms in to the right size (while not displaying the entire page width). This happens both in portrait and landscape mode, and the zoom level appears to be identical, or at least similar.
I have an alert come up telling me what the javascript has formatted body width and height to, and it is always right, but the browser feels the need to load as though the size of the window was actually larger.
I understand there's some nonsense where browsers get confused between CSS pixels and real pixels, but I thought the meta tag there was supposed to fix all of that.
So how do I make the browser load the page to a real 100% screen size, and not some sort of ~125% size?

Add
user-scalable=no
to your viewport meta-tag, this (at least in my tests) forces the browser to display at the width specified instead of the user/browsers preference.

Mobile Webpage with fixed width to support multiple devices

I came to a problem on the later stage of a project for a mobile page. Client asked to support both iPhone and various android mobile devices instead of supporting iPhone only.
The mobile page was written in XHTML, with html page width 640px, and its elements have width,e.g. 500px, %20... so the page look great in iPhone's safari with the following viewport, but ugly in other devices's browsers.
<meta name="viewport" content="user-scalable=no, width=device-width, target-densityDpi=device-dpi,initial-scale=0.5, maximum-scale=0.5, minimum-scale=0.5;">
I was trying to solve the problem by playing with viewport, but have being struggling for a long time. My idea is about the initial-scale, can someone suggest a way to dynamic assign this value = device-width/640 ?
Thanks

You should just use:
<meta name="viewport" content="width=320">
Why 320? It's because although the iPhone 4's screen is 640 pixels wide in portrait, it's a "retina" display with twice as high effective dpi as desktop screens. Hence it has a window.devicePixelRatio of 2, which means that its device-width is only 640/2 = 320 CSS px, i.e. each CSS px takes up a 2x2 square of physical device pixels (see this article on QuirksMode for more details).
This 2x scaling is very useful, because for example you can set font-size:14px and the perceived size of that text will be the same as if you viewed 14px text on an iPhone 3G (since it will actually be displayed using 28 physical device pixels on the iPhone 4, which compensates for the higher dpi). If instead your font-size:14px text was shown using only 14 physical device pixels, it would look tiny on the iPhone 4 (half the size that it would on a iPhone 3G).
Now, there is a consequence of this: your site has to be designed for 320px width, not 640px. But that was already true, since iOS has never supported target-densityDpi=device-dpi, so by putting width=device-width you were effectively putting width=320 anyway.
So why use width=320 instead of width=device-width? Since you say that your site is a fixed-width layout, then with width=device-width it is likely to look bad if it is shown on different size devices, for example you might see a white margin down the right hand side on wider devices, whereas with width=320 the browser will scale it up to fit the device's screen width (it may therefore look somewhat enlarged, but that's probably better than having a white margin).
However, please just consider this a stop-gap measure: it would be much better to keep width=device-width, and modify your site design to be flexible instead of fixed-width (for example set width:100% on your divs and images instead of width:320px). For more information, see http://www.html5rocks.com/en/mobile/responsivedesign/
Finally, you can keep the ", user-scalable=no" term if you really don't want the user to be able to zoom in and out, but for accessibility reasons it's often better to omit this, unless you're designing something like mobile maps.google.com, where you're manually handling pinch zoom gestures and you don't want to browser to interfere.