diff --git a/HTMLKit/CSSCodePoints.h b/HTMLKit/CSSCodePoints.h
index 93ae3ef..0d1a0fc 100644
--- a/HTMLKit/CSSCodePoints.h
+++ b/HTMLKit/CSSCodePoints.h
@@ -113,6 +113,25 @@ NS_INLINE BOOL isValidEscapedCodePoint(UTF32Char codePoint)
 			codePoint <= 0x10FFFF);
 }
 
+NS_INLINE BOOL isValidIdentifierStart(UTF32Char first, UTF32Char second, UTF32Char third)
+{
+	if (first == HYPHEN_MINUS) {
+		if (isNameStart(second) ||
+			second == HYPHEN_MINUS ||
+			isValidEscape(second, third)) {
+			return YES;
+		} else {
+			return NO;
+		}
+	} else if (isNameStart(first)) {
+		return YES;
+	} else if (first == REVERSE_SOLIDUS) {
+		return isValidEscape(first, second);
+	} else {
+		return NO;
+	}
+}
+
 NS_INLINE void AppendCodePoint(CFMutableStringRef string, UTF32Char codePoint)
 {
 	UniChar pair[2];
diff --git a/HTMLKit/CSSInputStream.m b/HTMLKit/CSSInputStream.m
index e9825a7..5a024bd 100644
--- a/HTMLKit/CSSInputStream.m
+++ b/HTMLKit/CSSInputStream.m
@@ -26,6 +26,12 @@
 {
 	CFMutableStringRef value = CFStringCreateMutable(kCFAllocatorDefault, 0);
 
+	if (!isValidIdentifierStart([self inputCharacterPointAtOffset:0],
+								[self inputCharacterPointAtOffset:1],
+								[self inputCharacterPointAtOffset:2])) {
+		return nil;
+	}
+
 	while (YES) {
 		UTF32Char codePoint = [self consumeNextInputCharacter];
 		if (codePoint == EOF) {