Your comments

Textastic uses the Discount Markdown library: http://www.pell.portland.or.us/~orc/Code/discount/

It supports Markdown and some extensions: I have compiled it with "--with-fenced-code" and "--enable-alpha-list".

Have a look at the Discount site to see what it supports.
Yes, you can tap on the action button and choose "Copy Generated HTML" to copy the HTML generated by the Discount Markdown library.

You can also directly send it as an HTML-formatted email by choosing "Send As Message" in Textastic.
Then you have to delete the app and reinstall it. 

Note: All your data and settings in Textastic are lost when you do that.
Apple's Mail app shows a light blue background for the current mail but doesn't select it when you tap on the "Edit" button. I think this would be a good solution. I'll consider adding this.
Hello,

before you tap on the "Edit" button in the Files section, the current file should be marked blue. This allows you to see which one is the current file, doesn't it?
Currently you have to manually keep track of the files you changed. There is currently no way to only download or upload changed files.

I usually use "Download Latest" before editing a file to make sure I'm working on the latest version.
Haben Sie die richtige Kodierung im HTML-Header angegeben?

Also z.B. 

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">