1 file changed, 20 insertions, 0 deletions
diff --git a/source/html.c b/source/html.c
index ce9344d..00eb7c6 100644
--- a/source/html.c
+++ b/source/html.c
@@ -471,6 +471,26 @@ void html_header_arg_in_quotes(const char *txt)
 		html(txt);
 }
 
+void html_header_ext_value(const char *txt)
+{
+	const char *p = txt;
+
+	while (p && *p) {
+		unsigned char c = *p;
+
+		// The attr-char set of RFC 8187, which leaves nothing that
+		// could end the parameter or the header line.
+		if (!isalnum(c) && !strchr("!#$&+-.^_`|~", c)) {
+			html_raw(txt, p - txt);
+			htmlf("%%%02X", c);
+			txt = p + 1;
+		}
+		p++;
+	}
+	if (p != txt)
+		html(txt);
+}
+
 /*
  * A query string goes into a header the way it arrived, already encoded by
  * the client, except that a byte a header line cannot carry is percent-encoded