Fix #10796. Allow IE<9 to retrieve uncomputed styles.

src/css.js

@@ -5,7 +5,7 @@ var ralpha = /alpha\([^)]*\)/i,
 	// fixed for IE9, see #8346
 	rupper = /([A-Z]|^ms)/g,
 	rnumpx = /^-?\d+(?:px)?$/i,
-	rnum = /^-?\d/,
+	rnumnopx = /^-?\d+(?!px)[^\d\s]+$/i,
 	rrelNum = /^([\-+])=([\-+.\de]+)/,
 
 	cssShow = { position: "absolute", visibility: "hidden", display: "block" },
@@ -280,7 +280,7 @@ if ( document.documentElement.currentStyle ) {
 
 		// Avoid setting ret to empty string here
 		// so we don't default to auto
-		if ( ret === null && style && (uncomputed = style[ name ]) ) {
+		if ( ret == null && style && (uncomputed = style[ name ]) ) {
 			ret = uncomputed;
 		}
 
@@ -289,7 +289,7 @@ if ( document.documentElement.currentStyle ) {
 
 		// If we're not dealing with a regular pixel number
 		// but a number that has a weird ending, we need to convert it to pixels
-		if ( !rnumpx.test( ret ) && rnum.test( ret ) ) {
+		if ( rnumnopx.test( ret ) ) {
 
 			// Remember the original values
 			left = style.left;

test/unit/css.js

@@ -511,6 +511,29 @@ test("can't get css for disconnected in IE<9, see #10254 and #8388", function()
 	equal( div.css( "top" ), "10px", "can't get top in IE<9, see #8388" );
 });
 
+test("can't get background-position in IE<9, see #10796", function() {
+	var div = jQuery( "<div/>" ).appendTo( "#qunit-fixture" ),
+		units = [
+			"0 0",
+			"12px 12px",
+			"13px 12em",
+			"12em 13px",
+			"12em center",
+			"+12em center",
+			"12.2em center",
+			"center center",
+		],
+		l = units.length,
+		i = 0;
+
+	expect( l );
+
+	for( ; i < l; i++ ) {
+		div.css( "background-position", units [ i ] );
+		ok( div.css( "background-position" ), "can't get background-position in IE<9, see #10796" );
+	}
+});
+
 test("Do not append px to 'fill-opacity' #9548", 1, function() {
 
 	var $div = jQuery("<div>").appendTo("#qunit-fixture");