diff --git a/src/parser/base.rs b/src/parser/base.rs
index 7b8c498..3d2cc5c 100644
--- a/src/parser/base.rs
+++ b/src/parser/base.rs
@@ -118,8 +118,7 @@ impl<'a> Parser<'a> {
 
         // If we do not find any characters that are not identifiers
         // then we are probably at the end of the stream
-        let end = simd::search_non_ident(bytes)
-            .unwrap_or_else(|| self.stream.len() - start);
+        let end = simd::search_non_ident(bytes).unwrap_or_else(|| self.stream.len() - start);
 
         self.stream.idx += end;
         Some(self.stream.slice(start, start + end))
@@ -219,10 +218,12 @@ impl<'a> Parser<'a> {
         self.stream.advance();
 
         let closing_tag_name = self.read_to(b'>');
-        
+
         self.stream.expect_and_skip_cond(b'>');
 
-        let closing_tag_matches_parent = self.stack.last()
+        let closing_tag_matches_parent = self
+            .stack
+            .last()
             .and_then(|last_handle| last_handle.get(self))
             .and_then(|last_item| last_item.as_tag())
             .map_or(false, |last_tag| last_tag.name() == closing_tag_name);
@@ -296,10 +297,9 @@ impl<'a> Parser<'a> {
             if simd::matches_case_insensitive(tag, *b"doctype") {
                 let doctype = self.read_ident()?;
 
-                let html5 = simd::matches_case_insensitive(doctype, *b"html");
-
-                if html5 {
-                    self.version = Some(HTMLVersion::HTML5);
+                if simd::matches_case_insensitive(doctype, *b"html") {
+                    self.skip_whitespaces();
+                    self.version = self.detect_html_version();
                 }
 
                 self.skip_whitespaces();
@@ -310,6 +310,36 @@ impl<'a> Parser<'a> {
         Some(())
     }
 
+    fn detect_html_version(&mut self) -> Option<HTMLVersion> {
+        if let Some(byte) = self.stream.current() {
+            if byte == &b'>' {
+                return Some(HTMLVersion::HTML5);
+            }
+        }
+
+        // it's old HTML 4 doctype. skipping the ' PUBLIC "-//W3C//DTD HTML 4.01"' part
+        self.stream.advance_by(29);
+
+        if let Some(byte) = self.stream.current() {
+            if byte == &b'/' {
+                return Some(HTMLVersion::StrictHTML401);
+            }
+        }
+
+        self.stream.advance();
+        let variant_marker = self.read_ident()?;
+
+        if simd::matches_case_insensitive(variant_marker, *b"transitional//en") {
+            return Some(HTMLVersion::TransitionalHTML401);
+        };
+
+        if simd::matches_case_insensitive(variant_marker, *b"frameset//en") {
+            return Some(HTMLVersion::FramesetHTML401);
+        };
+
+        None
+    }
+
     fn parse_tag(&mut self) -> Option<()> {
         let start = self.stream.idx;
 
diff --git a/src/simd/nightly.rs b/src/simd/nightly.rs
index 08e66ab..bb3d9e5 100644
--- a/src/simd/nightly.rs
+++ b/src/simd/nightly.rs
@@ -101,6 +101,9 @@ pub fn search_non_ident(haystack: &[u8]) -> Option<usize> {
     let needle_uc_z = u8x16::splat(b'Z');
     let needle_minus = u8x16::splat(b'-');
     let needle_underscore = u8x16::splat(b'_');
+    let needle_colon = u8x16::splat(b':');
+    let needle_plus = u8x16::splat(b'+');
+    let needle_slash = u8x16::splat(b'/');
 
     while i <= len - 16 {
         let mut bytes = [0; 16];
@@ -122,7 +125,10 @@ pub fn search_non_ident(haystack: &[u8]) -> Option<usize> {
 
         let eq_minus = bytes.simd_eq(needle_minus);
         let eq_underscore = bytes.simd_eq(needle_underscore);
-        let symbol = eq_minus | eq_underscore;
+        let eq_plus = bytes.simd_eq(needle_plus);
+        let eq_colon = bytes.simd_eq(needle_colon);
+        let eq_slash = bytes.simd_eq(needle_slash);
+        let symbol = eq_minus | eq_underscore | eq_plus | eq_colon | eq_slash;
 
         let or = !(digit | lowercase | uppercase | symbol).to_int();
 
diff --git a/src/tests.rs b/src/tests.rs
index 046861a..542a7fa 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -126,6 +126,30 @@ fn html5() {
     assert_eq!(dom.children().len(), 1)
 }
 
+#[test]
+fn html4_strict() {
+    let dom = parse(r#"<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> hello"#, ParserOptions::default()).unwrap();
+
+    assert_eq!(dom.version(), Some(HTMLVersion::StrictHTML401));
+    assert_eq!(dom.children().len(), 1)
+}
+
+#[test]
+fn html4_transitional() {
+    let dom = parse(r#"<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd"> hello"#, ParserOptions::default()).unwrap();
+
+    assert_eq!(dom.version(), Some(HTMLVersion::TransitionalHTML401));
+    assert_eq!(dom.children().len(), 1)
+}
+
+#[test]
+fn html4_frameset() {
+    let dom = parse(r#"<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/frameset.dtd"> hello"#, ParserOptions::default()).unwrap();
+
+    assert_eq!(dom.version(), Some(HTMLVersion::FramesetHTML401));
+    assert_eq!(dom.children().len(), 1)
+}
+
 #[test]
 fn ignore_void_closing_tags() {
     let input = r#"