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In the markdown, haven't attempted it but in theory it should work.

I just in:

<center>Hello World</center>

and went to preview and it shows. So looks like it works. I actually borrowed some code Steemit itself uses for post previews too :)

From screenshots I can say that it is wysiwyg html editor. So it should support most html tags. Although, @keverw should check if posts show up on Steemit as they are formatted on editor, because some html tags are stripped out of post body for security and XSS attacks reasons. Editor explore Part 1 and Part 2 talks about this a bit.