@keyframes FF 16+, IE 10, Opera 12.10; Chrome, Android, Safari 4, Opera 12, with prefix,
@import [url] list-of-media-queries the @import works currently only for [url], list-of-media-queries aids in RWD.
@page used to make printing style adjustment most useful for printable material.
While there are a few others they do not yet have widespread support and are considered experimental or highly experiemental
E[attr|="val"] attribute contains hyphenated values example span tag with lang attribute having value en-us
:lang(fr) `this one looks like it has some support in JSLint but is buggy when you have language code present it throws 'Expected ')' and instead saw 'fr'.' and when missing states expected language code.`
all 4 pseudo elements ::first-line ::first-letter ::before ::after
E ~ F sibling selector
CSS 3 Missing properties
Multiple values on
background-image & background-position, strangely background-origin already supports this in JSLint.
background-repeat, additionally the values round and space throw an error.
CSS 3 values
calc is either fully supported or supported with prefix. This one being quite useful for some styles of RWD.
hsl(0-360, 0-100%, 0-100%) & hsla(0-360, 0-100%, 0-100%, 0.0-1.0) both have support in modern browsers.
CSS Gradients (specs http://www.w3.org/TR/css3-images/#gradients)
rem units for length (spec http://www.w3.org/TR/css3-values/#rem-unit)
While prefixes are not an Ideal, they do allow for experimentation and presenting more advanced items where supported. FF finally adopted the -webkit- in conjunction with -moz- prefix; again graceful failure can be done here. Top 4 prefixes in use are -webkit-, -ms-, -moz-, -o- listed current market share order as listed in the use log of Wikimedia Oct 2012.
That looks like a good start. As time permits I will look for others that are currently missing support / under-supported / have errors.