This is a quirk of Live Preview. If you switch to Reading View (which renders more accurately), you’ll see that tag working. You can also use the tag:
operator to verify that Obsidian has indexed the tag.
You might post a feature request to make Live Preview recognize tags inside inline HTML, if a request doesn’t already exist. (Markdown only works in inline HTML, so altho hashtags aren’t Markdown I wouldn’t expect them to work in HTML blocks in Obsidian).
(Also, this doesn’t really matter for your notes but font
is a deprecated HTML element; I’d use span
.)