Expanding to @Chris to respond to the semantics located there if you are interested in the visible effect of the font inside your notes, you have three approaches:
Select parts of the text.

Nesting works well if you do not need to start / end two tags at once.
Use multiple tags with escape characters
Closest you can get

Code: *\ /\ _\ ~fontification can be nested~\_\/\*
So, you need \ backslash and space) to avoid the following opening tags and \ (backslash) to avoid the following closing tags.
The need for space is annoying, and it looks like this when exported to html: 
So, you can have several allowances at once, but you need to choose between viewing the effect in emacs or its beautiful design when exporting.
Change the layout of emacs
Alternatively, you can change how markup looks in emacs without changing the export format, i.e. in order to make bold red you will need the following:
(add-to-list 'org-emphasis-alist '("*" (:foreground "red") ))
as described in this wonderful question and answer .
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