diff --git a/lib/md033.js b/lib/md033.js
index 331c70a3..6bfa0516 100644
--- a/lib/md033.js
+++ b/lib/md033.js
@@ -6,7 +6,7 @@ const { addError, bareUrlRe, forEachLine, unescapeMarkdown } =
   require("../helpers");
 const { lineMetadata } = require("./cache");
 
-const htmlElementRe = /<(([\w-]+)(?:[\s/][^>]*)?)>/g;
+const htmlElementRe = /<(([A-Za-z][A-Za-z0-9-]*)(?:[\s/][^>]*)?)>/g;
 const linkDestinationRe = /]\(\s*$/;
 const inlineCodeRe = /^[^`]*(`+[^`]+`+[^`]+)*`+[^`]*$/;
 // See https://spec.commonmark.org/0.29/#autolinks
diff --git a/test/detailed-results-html-tags.md b/test/detailed-results-html-tags.md
index 86159a0b..0bbf4773 100644
--- a/test/detailed-results-html-tags.md
+++ b/test/detailed-results-html-tags.md
@@ -101,5 +101,7 @@ Text
 <foo-bar-baz> is an HTML element.
 
 But <foo.bar.baz> is not an autolink or HTML element.
+And neither is <foo_bar>.
+Nor <123abc>.
 
 Text