I uppercased the region (US) in the language tag to match the BCP-47 format.

Daniel Arthur Gallagher · Daniel Arthur Gallagher · commit 9dc92cd7afed · 2017-06-26T14:38:43.000+01:00
diff --git a/index.html b/index.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="en-us" dir="ltr">
+<html lang="en-US" dir="ltr">
 <head>
   <title>Scriptular - Javascript Regular Expression Editor</title>
   <link href="application.css" rel="stylesheet">
