In Tumblr’s Preferences screen, you can specify Markdown as your editor instead of the automatic HTML-generating WYSIWYG view:
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The only downside is that it fills up with HTML on reblogs, so you have to be a bit of an HTML geek if you reblog a lot. But give it a try: you might like it.
I’ve often wondered why this is the case. The Markdown doesn’t get immediately translated to HTML because edits to your own Markdown posts stay in Markdown. I suppose it’s because you can’t guarantee everyone else’s posts will be in Markdown, so the reblog just drops to the lowest-common-denominator of HTML.
It’d be super cool if posts were run through Markdownify or similar so that reblogs could be in Markdown too.