Miraculous :checked pseudo-class polyfill (for IE8 and below).

Include jQuery 1.x (compatible with old IE) >= 1.7, then the poly-checked script inside an IE conditional comment:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<!--[if lte IE 8]>
<script src="proxy.php?url=https%3A%2F%2Fgithub.com%2Fpath%2Fto%2Fpoly-checked.min.js"></script>
<![endif]-->

Make sure to include all stylesheets before the script and that's all!

See live demo.

• Transparent to the developer, no need to add ugly polyfill classes to your CSS.
• Works with dynamically created checkbox/radio elements.
• Full interoperability with JavaScript. Like the real :checked pseudo-class, changing an element's checked state will automatically update its style.
• Keeps the same selector specifity as your original selectors.
• Bonus: extra patch for :checked to work with sibling selectors. Very useful for customized CSS checkboxes/radios.

Currently tested in IE 7 and 8.

• Can't polyfill stylesheets from different domains. There's no CSSOM/DOM API to retrieve unrecognized selectors nor raw CSS text from a <link rel="stylesheet"> element, hence the polyfill uses XHR which is subject to the same-origin policy. The workaround is to host all stylesheets that require polyfilling in the same domain. Feel free to submit a PR or open an issue for discussion if you find a better alternative.
• Detecting dynamically added elements is done through polling, which may cause some FOUC. jQuery's .clone(true) can be used on an already patched checkbox/radio to avoid the milliseconds of FOUC. It'd best if we could get rid of the polling altogether, but I haven't found any evented way to detect newly created input elements (in IE<=8). Feel free to open a PR/issue as usual if there's any better way to handle this in old IE.
• Does not polyfill dynamically added stylesheets. This is a rare use case and I don't feel like adding more DOM polling here, but it is very possible to implement a polyCheckedParseNewStylesheets function. Feel free to open a PR/issue to discuss it if you need this functionality.
• Does not polyfill the :checked pseudo-class for option elements (option:checked). Option elements are rather limited and cross-browser inconsistent for styling, hence this is not one of my priorities. You can always use one of the thousands of styled select/combobox plugins available out there.