diff --git a/_test/extra.txt b/_test/extra.txt
index a1e2c9a..f8d89e6 100644
--- a/_test/extra.txt
+++ b/_test/extra.txt
@@ -392,8 +392,11 @@ a* b c d *e*
     OPTIONS: {"enableEscape": true}
 //- - - - - - - - -//
 hello\x00world
+
+<?\x00
 //- - - - - - - - -//
 <p>hello\ufffdworld</p>
+<?\uFFFD
 //= = = = = = = = = = = = = = = = = = = = = = = =//
 
 27: Newlines in code spans must be preserved as a space
diff --git a/renderer/html/html.go b/renderer/html/html.go
index c1b0d21..7f5b9fe 100644
--- a/renderer/html/html.go
+++ b/renderer/html/html.go
@@ -296,7 +296,7 @@ func (r *Renderer) renderHTMLBlock(w util.BufWriter, source []byte, node ast.Nod
 			l := n.Lines().Len()
 			for i := 0; i < l; i++ {
 				line := n.Lines().At(i)
-				_, _ = w.Write(line.Value(source))
+				r.Writer.SecureWrite(w, line.Value(source))
 			}
 		} else {
 			_, _ = w.WriteString("<!-- raw HTML omitted -->\n")
@@ -305,7 +305,7 @@ func (r *Renderer) renderHTMLBlock(w util.BufWriter, source []byte, node ast.Nod
 		if n.HasClosure() {
 			if r.Unsafe {
 				closure := n.ClosureLine
-				_, _ = w.Write(closure.Value(source))
+				r.Writer.SecureWrite(w, closure.Value(source))
 			} else {
 				_, _ = w.WriteString("<!-- raw HTML omitted -->\n")
 			}
@@ -668,8 +668,13 @@ type Writer interface {
 	// RawWrite writes the given source to writer without resolving references and
 	// unescaping backslash escaped characters.
 	RawWrite(writer util.BufWriter, source []byte)
+
+	// SecureWrite writes the given source to writer with replacing insecure characters.
+	SecureWrite(writer util.BufWriter, source []byte)
 }
 
+var replacementCharacter = []byte("\ufffd")
+
 type defaultWriter struct {
 }
 
@@ -684,6 +689,23 @@ func escapeRune(writer util.BufWriter, r rune) {
 	_, _ = writer.WriteRune(util.ToValidRune(r))
 }
 
+func (d *defaultWriter) SecureWrite(writer util.BufWriter, source []byte) {
+	n := 0
+	l := len(source)
+	for i := 0; i < l; i++ {
+		if source[i] == '\u0000' {
+			_, _ = writer.Write(source[i-n : i])
+			n = 0
+			_, _ = writer.Write(replacementCharacter)
+			continue
+		}
+		n++
+	}
+	if n != 0 {
+		_, _ = writer.Write(source[l-n:])
+	}
+}
+
 func (d *defaultWriter) RawWrite(writer util.BufWriter, source []byte) {
 	n := 0
 	l := len(source)
@@ -719,7 +741,7 @@ func (d *defaultWriter) Write(writer util.BufWriter, source []byte) {
 		}
 		if c == '\x00' {
 			d.RawWrite(writer, source[n:i])
-			d.RawWrite(writer, []byte("\ufffd"))
+			d.RawWrite(writer, replacementCharacter)
 			n = i + 1
 			continue
 		}