Markdown heading levels not working

My previously filed ticket seems to be not saved, thus just as a backup here agin (please mark as duplicate if I missed something).

The following Markdown

H1
=

H2
==

H3
===

H4
====

yields this result:

<!DOCTYPE html>
<html>
<head>
<style>body > img, blockquote >img { max-width: 100%; }
body { font-family: "Menlo"; font-size: 8.2pt; }
</style>
<meta http-equiv="x-dns-prefetch-control" content="off">
<meta http-equiv="Content-Type" content="text/xhtml; charset=utf-8">
<style>
div.markdown { white-space: normal; }
body { font-family: sans-serif; }
h1 { font-size: 1.4em; }
h2 { font-size: 1.2em; }
h3 { font-size: 1.1em; }

...
</style>
</head>
<body caret_uuid="D348A516-0ABC-4674-AFED-2A7B64EBB756"><div class="markdown">
<h1 data-sourcepos="1:1-3:0">H1</h1>
<h1 data-sourcepos="4:1-6:0">H2</h1>
<h1 data-sourcepos="7:1-9:0">H3</h1>
<span D348A516-0ABC-4674-AFED-2A7B64EBB756="0.500000" style="display:inline "></span><h1 data-sourcepos="10:1-11:4">H4</h1>

</div></body>

</html>

Note that the header levels are not correctly converted to HTML header levels. Also h4-h6 are not converted properly (but I guess they're part of both Markdown and HTML).