So, you can certainly use Deepdwn to write your markdown for Nikola, and it won’t prevent you from adding arbitrary HTML tags and attributes to your document, but its preview isn’t going to interpret the markdown in the same way that Nikola/Python-MarkdownExtra does.
For instance,
<SECTION markdown="1">
## A level two heading
A paragraph with **bold** and *italics*.
</SECTION>
Will be interpreted the same as
<SECTION>
## A level two heading
A paragraph with **bold** and *italics*.
</SECTION>
Because Deepdwn doesn’t consider the markdown attribute on the section at all.
Also note the empty line after the section node, this is required in commonmark to interpret the following lines as markdown, instead of more HTML.