diff --git a/HTMLKit.xcodeproj/project.pbxproj b/HTMLKit.xcodeproj/project.pbxproj
index f7dbba3..4ce9ffa 100644
--- a/HTMLKit.xcodeproj/project.pbxproj
+++ b/HTMLKit.xcodeproj/project.pbxproj
@@ -119,6 +119,7 @@
 		62362A3A1A9FA70400301989 /* HTMLText.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = HTMLText.h; sourceTree = "<group>"; };
 		62362A3B1A9FA70400301989 /* HTMLText.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = HTMLText.m; sourceTree = "<group>"; };
 		62362A3F1A9FDE8A00301989 /* HTMLNodes.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = HTMLNodes.h; sourceTree = "<group>"; };
+		623719431AA12EE8002E03C8 /* HTMLQuirksMode.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = HTMLQuirksMode.h; sourceTree = "<group>"; };
 		6238578F1A9E772B003A45D9 /* HTMLDocument.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = HTMLDocument.h; sourceTree = "<group>"; };
 		623857901A9E772B003A45D9 /* HTMLDocument.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = HTMLDocument.m; sourceTree = "<group>"; };
 		623857941A9E8606003A45D9 /* HTMLDocumentType.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = HTMLDocumentType.h; sourceTree = "<group>"; };
@@ -272,6 +273,7 @@
 				6223211D1A969B9300BACED5 /* HTMLElementTypes.h */,
 				6279F87119E17DC700F12EE5 /* HTMLParserInsertionModes.h */,
 				62362A3F1A9FDE8A00301989 /* HTMLNodes.h */,
+				623719431AA12EE8002E03C8 /* HTMLQuirksMode.h */,
 				6234584C1A9D2FA4009BD491 /* HTMLNode.h */,
 				6234584D1A9D2FA4009BD491 /* HTMLNode.m */,
 				6279F87219E1808D00F12EE5 /* HTMLElement.h */,
diff --git a/HTMLKit/HTMLDocument.h b/HTMLKit/HTMLDocument.h
index 4ce662c..cc4e1b3 100644
--- a/HTMLKit/HTMLDocument.h
+++ b/HTMLKit/HTMLDocument.h
@@ -8,13 +8,7 @@
 
 #import "HTMLNode.h"
 #import "HTMLDocumentType.h"
-
-typedef NS_ENUM(short, HTMLQuirksMode)
-{
-	HTMLQuirksModeNoQuirks,
-	HTMLQuirksModeQuirks,
-	HTMLQuirksModeLimitedQuirks
-};
+#import "HTMLQuirksMode.h"
 
 @interface HTMLDocument : HTMLNode
 
diff --git a/HTMLKit/HTMLDocumentType.h b/HTMLKit/HTMLDocumentType.h
index f02d42e..d872730 100644
--- a/HTMLKit/HTMLDocumentType.h
+++ b/HTMLKit/HTMLDocumentType.h
@@ -7,6 +7,7 @@
 //
 
 #import "HTMLNode.h"
+#import "HTMLQuirksMode.h"
 
 @interface HTMLDocumentType : HTMLNode
 
@@ -18,4 +19,7 @@
 			publicIdentifier:(NSString *)publicIdentifier
 			systemIdentifier:(NSString *)systemIdentifier;
 
+- (BOOL)isValid;
+- (HTMLQuirksMode)quirksMode;
+
 @end
diff --git a/HTMLKit/HTMLDocumentType.m b/HTMLKit/HTMLDocumentType.m
index 748f550..3a07c98 100644
--- a/HTMLKit/HTMLDocumentType.m
+++ b/HTMLKit/HTMLDocumentType.m
@@ -8,6 +8,10 @@
 
 #import "HTMLDocumentType.h"
 
+NS_INLINE BOOL nilOrEqual(id first, id second) {
+	return (first == nil) || ([first isEqual:second]);
+}
+
 @implementation HTMLDocumentType
 
 - (instancetype)initWithName:(NSString *)name
@@ -22,6 +26,86 @@
 	return self;
 }
 
+- (BOOL)isValid
+{
+	if (![self.name isEqualToString:@"html"]) {
+		return NO;
+	}
+
+	if ([_publicIdentifier isEqualToString:@"-//W3C//DTD HTML 4.0//EN"] &&
+		nilOrEqual(_systemIdentifier, @"http://www.w3.org/TR/REC-html40/strict.dtd")) {
+		return YES;
+	}
+
+	if ([_publicIdentifier isEqualToString:@"-//W3C//DTD HTML 4.01//EN"] &&
+		nilOrEqual(_systemIdentifier, @"http://www.w3.org/TR/html4/strict.dtd")) {
+		return YES;
+	}
+
+	if ([_publicIdentifier isEqualToString:@"-//W3C//DTD XHTML 1.0 Strict//EN"] &&
+		nilOrEqual(_systemIdentifier, @"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd")) {
+		return YES;
+	}
+
+	if ([_publicIdentifier isEqualToString:@"-//W3C//DTD XHTML 1.1//EN"] &&
+		nilOrEqual(_systemIdentifier, @"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd")) {
+		return YES;
+	}
+
+	if (_publicIdentifier != nil) {
+		return NO;
+	}
+
+	if (_systemIdentifier && ![_systemIdentifier isEqualToString:@"about:legacy-compat"]) {
+		return NO;
+	}
+
+	return YES;
+}
+
+- (HTMLQuirksMode)quirksMode
+{
+	if (![self.name isEqualToString:@"html"]) {
+		return HTMLQuirksModeQuirks;
+	}
+
+	if (isEqualCaseInsensitive(_publicIdentifier, @"-//W3O//DTD W3 HTML Strict 3.0//EN//") ||
+		isEqualCaseInsensitive(_publicIdentifier, @"-/W3C/DTD HTML 4.0 Transitional/EN") ||
+		isEqualCaseInsensitive(_publicIdentifier, @"HTML")) {
+		return HTMLQuirksModeQuirks;
+	}
+	
+	if (isEqualCaseInsensitive(_systemIdentifier, @"http://www.ibm.com/data/dtd/v11/ibmxhtml1-transitional.dtd")) {
+		return HTMLQuirksModeQuirks;
+	}
+
+	if (QuirksModePrefixMatch(_publicIdentifier)) {
+		return HTMLQuirksModeQuirks;
+	}
+
+	if (_systemIdentifier == nil) {
+		if (hasPrefixCaseInsensitive(_publicIdentifier, @"-//W3C//DTD HTML 4.01 Frameset//") ||
+			hasPrefixCaseInsensitive(_publicIdentifier, @"-//W3C//DTD HTML 4.01 Transitional//")) {
+			return HTMLQuirksModeQuirks;
+		}
+	}
+
+#warning Check "iframe srcdoc"
+	if (hasPrefixCaseInsensitive(_publicIdentifier, @"-//W3C//DTD XHTML 1.0 Frameset//") ||
+		hasPrefixCaseInsensitive(_publicIdentifier, @"-//W3C//DTD XHTML 1.0 Transitional//")) {
+		return HTMLQuirksModeLimitedQuirks;
+	}
+
+	if (_systemIdentifier != nil) {
+		if (hasPrefixCaseInsensitive(_publicIdentifier, @"-//W3C//DTD HTML 4.01 Frameset//") ||
+			hasPrefixCaseInsensitive(_publicIdentifier, @"-//W3C//DTD HTML 4.01 Transitional//")) {
+			return HTMLQuirksModeLimitedQuirks;
+		}
+	}
+
+	return HTMLQuirksModeNoQuirks;
+}
+
 #pragma mark - NSCopying
 
 - (id)copyWithZone:(NSZone *)zone
diff --git a/HTMLKit/HTMLQuirksMode.h b/HTMLKit/HTMLQuirksMode.h
new file mode 100644
index 0000000..e5caf19
--- /dev/null
+++ b/HTMLKit/HTMLQuirksMode.h
@@ -0,0 +1,97 @@
+//
+//  HTMLQuirksMode.h
+//  HTMLKit
+//
+//  Created by Iska on 28/02/15.
+//  Copyright (c) 2015 BrainCookie. All rights reserved.
+//
+
+typedef NS_ENUM(short, HTMLQuirksMode)
+{
+	HTMLQuirksModeNoQuirks,
+	HTMLQuirksModeQuirks,
+	HTMLQuirksModeLimitedQuirks
+};
+
+#define QUIRKS_MODE_PREFIXES \
+	QUIRKS_ENTRY( "+//Silmaril//dtd html Pro v0r11 19970101//" ) \
+	QUIRKS_ENTRY( "-//AS//DTD HTML 3.0 asWedit + extensions//" ) \
+	QUIRKS_ENTRY( "-//AdvaSoft Ltd//DTD HTML 3.0 asWedit + extensions//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.0 Level 1//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.0 Level 2//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.0 Strict Level 1//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.0 Strict Level 2//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.0 Strict//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.0//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 2.1E//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 3.0//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 3.2 Final//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 3.2//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML 3//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Level 0//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Level 1//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Level 2//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Level 3//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Strict Level 0//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Strict Level 1//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Strict Level 2//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Strict Level 3//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML Strict//" ) \
+	QUIRKS_ENTRY( "-//IETF//DTD HTML//" ) \
+	QUIRKS_ENTRY( "-//Metrius//DTD Metrius Presentational//" ) \
+	QUIRKS_ENTRY( "-//Microsoft//DTD Internet Explorer 2.0 HTML Strict//" ) \
+	QUIRKS_ENTRY( "-//Microsoft//DTD Internet Explorer 2.0 HTML//" ) \
+	QUIRKS_ENTRY( "-//Microsoft//DTD Internet Explorer 2.0 Tables//" ) \
+	QUIRKS_ENTRY( "-//Microsoft//DTD Internet Explorer 3.0 HTML Strict//" ) \
+	QUIRKS_ENTRY( "-//Microsoft//DTD Internet Explorer 3.0 HTML//" ) \
+	QUIRKS_ENTRY( "-//Microsoft//DTD Internet Explorer 3.0 Tables//" ) \
+	QUIRKS_ENTRY( "-//Netscape Comm. Corp.//DTD HTML//" ) \
+	QUIRKS_ENTRY( "-//Netscape Comm. Corp.//DTD Strict HTML//" ) \
+	QUIRKS_ENTRY( "-//O'Reilly and Associates//DTD HTML 2.0//" ) \
+	QUIRKS_ENTRY( "-//O'Reilly and Associates//DTD HTML Extended 1.0//" ) \
+	QUIRKS_ENTRY( "-//O'Reilly and Associates//DTD HTML Extended Relaxed 1.0//" ) \
+	QUIRKS_ENTRY( "-//SQ//DTD HTML 2.0 HoTMetaL + extensions//" ) \
+	QUIRKS_ENTRY( "-//SoftQuad Software//DTD HoTMetaL PRO 6.0::19990601::extensions to HTML 4.0//" ) \
+	QUIRKS_ENTRY( "-//SoftQuad//DTD HoTMetaL PRO 4.0::19971010::extensions to HTML 4.0//" ) \
+	QUIRKS_ENTRY( "-//Spyglass//DTD HTML 2.0 Extended//" ) \
+	QUIRKS_ENTRY( "-//Sun Microsystems Corp.//DTD HotJava HTML//" ) \
+	QUIRKS_ENTRY( "-//Sun Microsystems Corp.//DTD HotJava Strict HTML//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 3 1995-03-24//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 3.2 Draft//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 3.2 Final//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 3.2//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 3.2S Draft//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 4.0 Frameset//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML 4.0 Transitional//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML Experimental 19960712//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD HTML Experimental 970421//" ) \
+	QUIRKS_ENTRY( "-//W3C//DTD W3 HTML//" ) \
+	QUIRKS_ENTRY( "-//W3O//DTD W3 HTML 3.0//" ) \
+	QUIRKS_ENTRY( "-//WebTechs//DTD Mozilla HTML 2.0//" ) \
+	QUIRKS_ENTRY( "-//WebTechs//DTD Mozilla HTML//" )
+
+static NSString * HTMLQuirksModePrefixes[] = {
+#define QUIRKS_ENTRY( prefix ) @prefix,
+	QUIRKS_MODE_PREFIXES
+#undef QUIRKS_ENTRY
+};
+
+NS_INLINE BOOL isEqualCaseInsensitive(NSString *first, NSString *second) {
+	return [first caseInsensitiveCompare:second] == NSOrderedSame;
+}
+
+NS_INLINE BOOL hasPrefixCaseInsensitive(NSString *string, NSString *prefix) {
+	NSRange reange = [string rangeOfString:prefix
+								   options:NSAnchoredSearch|NSCaseInsensitiveSearch];
+	return reange.location != NSNotFound;
+}
+
+NS_INLINE BOOL QuirksModePrefixMatch(NSString *publicIdentifier)
+{
+	for (int i = 0; i < sizeof(HTMLQuirksModePrefixes) / sizeof(HTMLQuirksModePrefixes[0]); i++) {
+		if (hasPrefixCaseInsensitive(publicIdentifier, HTMLQuirksModePrefixes[i])) {
+			return YES;
+		}
+	}
+	return NO;
+}