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Selectively Support Certain Browsers

Ok everyone, I have a requirement to only support IE 11+, Chrome 31+, etc. The specifics don't really matter at this point. I know it's not ideal to restrict other browsers, but this is a vendor requirement and isn't my call. I'm just trying to figure out how to go about doing this.
Initially I wanted to use feature detection (my assumption was that the site needed to be HTML5 compatible). These requirements have changed. Is feature detection still viable for this? I would prefer to not use user agent sniffing, since it's so easy to spoof.
I'm using ASP.NET, C#, .NET 4.0, jQuery, HTML5, CSS3
How can I accomplish this?
Thanks in advance!

You can look inside the Request server variable for useful information about the browser. While debugging, it looks to me like this is getting generated from the user agent. From what I know about the web though, this is how clients communicate to servers what they are capable of viewing.
There are a few properties that can help you within Request.Browser.
Request.Browser.Type //Returns "Chrome41"
Request.Browser.Browser // Returns "Chrome"
You could also use conditional comments for IE, something like this:
<!--[if IE 11]>
(Redirect users here or however you want to handle it)
<![endif]-->
I'm sure you've seen these before, but that statement basically is targeting any IE version less than 11. These are Microsoft / IE specific so while this will help you with your IE issue, you will most likely have to rely on the user agent or Request as above for other browsers.

How can I reliably detect the browser without using window.navigator?

I know there are a thousand questions on Stack Overflow about detecting the browser with JavaScript. My question is how can you detect the browser without window.navigator (which includes navigator.userAgent)?
First, to clarify, I don't need to know the rendering engine, this isn't for adaptive layout, and don't panic: I'm already doing feature detection. If why I'm asking about detecting the browser is important, please comment and I'll be happy to splice in the explanation, but it will probably make the question egregiously long.
Next, let me describe why my question is not a duplicate of:
Browser detection in Javascript? because of 19 answers, 12 of them use navigator.userAgent specifically (including jQuery.browser which used userAgent, and is now gone anyway), 4 use navigator.appName (which gives "Netscape" in Chrome...), 1 side-steps the question by recommending feature detection, which is different from browser detection (I am already using feature detection, but to know the extent to which I can use them, I need browser detection), and 2 aren't really answers or are IE-specific. (Although this non-answer is actually very explanatory about why my question here is relevant: I'm trying to avoid hitting pain points on certain browsers that would crash the tab!) Since my question is asking for an answer (even a hack?) without using window.navigator, it is not a duplicate of that question.
Check if the user is using IE because of 11 answers, 10 use navigator.userAgent and 1 of them uses an IE trick to detect IE only, which is not sufficient to answer my question (though it may be potentially be a small part of a helpful solution posted here)?
In Javascript, how do I determine if my current browser is Firefox on a computer vs everything else? because of 11 answers, 8 use navigator.userAgent, 2 recommend feature detection (again, not my question), and 1 isn't even an answer, really.
How to detect chrome and safari browser (webkit) because of 8 answers, 6 of them use navigator.userAgent, and 2 are webkit-specific. Unfortunately, WebKit is not necessarily tied to just Safari, and I need to know the browser, not the rendering engine.
Hopefully that is crystal clear.
I know there are other ways to do this, but I don't know the ins-and-outs of each browser well enough. Are there objects or variables that are consistently or reliably exposed to JavaScript in certain browsers, maybe? I know that some experimental APIs are vendor-prefixed, but that doesn't seem like a good idea for use in a commercial product, although I'm willing to stoop that low if needed. Any other possibilities?

A thought:
IE uses ActiveX (still does up to IE11, legacy), You can fairly easy deduce the fact that the user is using IE from looking at activeX availability, however if the security settings are on, you need to fall back on, guess what, other feature detection.
Chrome and Firefox both support the use of extensions, maybe detecting these extensions will help
Chrome has the window['chrome']['webstore'] object available in the global scope.
You can sort through the window object with Object.keys and look for vendor specific names like 'moz' or 'ms' or 'o'.
If you combine moz, ms and chrome-object you can sniff out the three largest browsers.
On a side note, feature detection is still the best option, not for the OP, but for the "I'm-just-getting-into-programming-and-I-like-to-know-how-I-sniff-out-a-browser-programmers" out there.

First off, just to be clear no inbound HTTP request is guaranteed to be accurate in saying who they are. In fact this is how some hackers operate by faking out the User-Agent field of the HTTP header. Most browser providers; however, do a good job of identifying who they are by saying so in the User-agent field of the inbound HTTP request header. The only problem working at the Javascript layer is that it is one layer too high for seeing the headers. However there are tricks and here's one of them :
var req = new XMLHttpRequest();
req.open('GET', document.location, false);
req.send(null);
var headers = req.getAllResponseHeaders().toLowerCase();
alert(headers);
I found this after thinking about the problem a bit and knowing the Network side pretty well... You can read more about this here: Accessing the web page's HTTP Headers in JavaScript, in fact this question could be considered a duplicate of that answer, however, not everyone knows the HTTP layer. The code above is pretending to be a legitimate inbound request, and opens the document.location using a "Get", but it sends nothing. Without knowing the browser internals I am guessing that it is the DOM itself that is returning the already known header information. Just parse the User-Agent portion of he response and you are all set.

Force Employee To Use Firefox + Specified Version For Company Intranet

Please not not turn this into a discussion about which browser is better and the ethics of forcing a browser. It's an intranet, and it's what I am required to do so everyone calm down =o)
I need to prevent employees from trying to bypass the check to not use their preferred browser instead of the company mandated one + version. The decision was made based on security, compatibility, costs, and the use of company made Firefox extensions.
I came across this Force users to use Chrome/FireFox instead of IE which I can do easily in PHP to force use of Firefox, however it relies on the useragent which can easily be changed in numerous browsers and with the use of plugins.
Is there a JavaScript solution that I can use that DOES NOT check the useragent or any value that can be 'easily' modified by a user/plugin? It would need to detect if the browser is Firefox and what version it is. Site uses jQuery so if it can be done using that, however not required then by all means yes. I just am not aware of what ways to detect a browser and it's version that there are without checking useragent.
I remember way back in the day for detecting Netscape or some browser checking for document.all was used instead of useragent, so I'm guessing something similar, which only Firefox will have.
Thank you very much in advance!

Try this: http://jsfiddle.net/DerekL/62exH/
Because Firefox handles onpopstate differently than other browsers, you can then use this to detect if it is Firefox or not. This can not be changed by user or script. But the disadvantage is you can only get the version number by doing navigator.appVersion.

You can only try but cannot succeed in forcing a browser. That being said you can strip down the CSS in other browsers which may completely make your site close to unusable in other browsers.
To make your CSS only work with Firefox you can try approaches given # Targeting only Firefox with CSS

Can IE 9 still sign with Javascript like in old IE and Netscape / Firefox?

It was possible to sign with digital certificate in IE and Netscape
http://bozhobg.wordpress.com/2009/04/16/how-to-create-a-digital-signing-solution-with-only-javascript/
What's the equivalent in IE 9 ?

You're going to have a problem with this.
The clue is in the code in the link you provided. Specifically, where it uses new ActiveXObject().
ActiveX is a very old technology and it has severe security issues. For this reason, it's use has been discouraged for some time (this was the case a long time before the article you linked to was written).
IE9 does still support it, but only for legacy reasons; its use is strongly discouraged, and you will need to go to the browser config and disable some security settings in order to get it working.
If you do get activeX working in IE9, you'll also need to make sure you have the relevant activeX controls installed on your PC that actually do the work (I've not used the ones in question, so I can't advise on them). In addition, since the activeX technology is deprecated, you may find that the activeX control you need to use may not have been kept up-to-date. This may affect whether it works with newer versions of IE or Windows.

Best way to display warning message to "untested" browsers?

I have an app that I have fully tested in Safari 5, IE 9, FF6, and Chrome 14. I'm not looking to block or exclude any browsers from the experience. But I want to warn/inform users that there may be a better experience in another browser, and if they choose to continue, there may be features not working or broken.
I have looked at jQuery browser detection, but it seems to be a bit quirky. I know the best solution would be to warn based on feature detection, but we are currently in beta and I am not completely sure what features make or break. Such as web workers, its known that web workers not working breaks our app, but it works in IE lower versions. But then there is an issue with Opera that web workers is available, but not functioning correctly.
Ultimately I am looking for an easy way to say Browser X Version y and up don't show warning, and those and under show warning.
What is the best way to approach this?

Browser detection is indeed "a bit quirky", in pretty much any scenario. The jQuery route is probably as good as you're going to get, but as you say it's not great.
A better solution is generally to do feature detection, especially in cases like the one you describe where your site relies on specific features.
The best feature detection library is Modernizr. This will give you an bunch of Javascript flags which are set to true or false according to whether the browser supports a given feature. It detects support for a whole bunch of stuff, including Web Workers.

Check out this: http://www.w3schools.com/js/js_browser.asp
-Easy way of detecting the user's browser with javascript. From there I'd just use a switch statement or something to display the messages for browsers that aren't tested yet. If you want the exact browser version you'll have to parse it from the "navigator.userAgent" field.

If the goal is full validation, you need to be even more specific about versions. Keep in mind that some browser upgrades are not 100% backwards compatible with previous versions. (Look at how IE8 mode in IE9 is not the same as native IE8 rendering, for instance.) You're going to have to retest with every new browser version, and sooner or later there's going to be a "Fully tested with browser version X, not tested with version X+1 that was released yesterday" problem. Feature detection, graceful degradation, and a warning non-intrusively displayed to the user if their experience is being significantly downgraded is a better way to go.
To directly answer your question, if you must implement what you are asking for just parse the user agent. This could be useful specifically watching out for a browser you know doesn't work right and warning, or as a bandaid for a badly written intranet app that is very picky about the exact browser version it will run on. For a newly developed app where you have control over the requirements, I would not recommend warning on browser version since there are better ways to do it.