GFM enables the tagfilter extension, where the following HTML tags will be
filtered when rendering HTML output:
<title>
<textarea>
<style>
<xmp>
<iframe>
<noembed>
<noframes>
<script>
<plaintext>
Filtering is done by replacing the leading <
with the entity <
. These
tags are chosen in particular as they change how HTML is interpreted in a way
unique to them (i.e. nested HTML is interpreted differently), and this is
usually undesireable (sic) in the context of other rendered Markdown content.
All other HTML tags are left untouched.