--- a/src/html.cpp	Tue Feb 25 17:51:50 2025 +0100
+++ b/src/html.cpp	Tue Feb 25 17:56:54 2025 +0100
@@ -72,7 +72,8 @@
         puts("<div class=\"heatmap-content\">");
         indentation = 1;
     } else {
-        puts(R"(<html>
+        puts(R"(<!DOCTYPE html>
+<html>
     <head>
         <style>
             table.heatmap {