Your comments

Please try to enable the "Debug Log" setting and have a look at the log file.

What is the exact error message?
For syntax highlighting, there is a also a DocBook TextMate bundle you can try at https://github.com/weppos/docbook5-tmbundle

Here's how to use custom TextMate bundles with Textastic: http://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html
Did you see the following entry regarding Docbook preview?

http://feedback.textasticapp.com/topic/80566-how-can-i-preview-docbook-files-in-textastic/

Thanks for reporting this. I'll try to reproduce and fix it.
Here is the xml stylesheet processing instruction I used:

<?xml-stylesheet type="text/xsl" href="docbook-xsl-1.76.1/html/docbook.xsl" ?>
You can download the xsl stylesheets from http://sourceforge.net/projects/docbook/files/docbook-xsl/

If you want "html" output, you only need the "common", "html", "lib" directories and the "VERSION" file as you can see in the last screenshot.
You can use Textastic's preview feature with DocBook files if you copy the DocBook xsl stylesheets to Textastic and add a <?xml-stylesheet?> processing instruction:




Yes there are plans. But I can't tell when it is going to be done. 

Btw: this thread was started almost immediately after Textastic 1.0 was released. Before it had Dropbox, (S)FTP and the new syntax highlighter. I think Textastic gained a massive amount of features since then.
The symbols are defined in the TextMate bundles that are included in Textastic.

This is documented in the TextMate manual: http://manual.macromates.com/en/navigation_overview#customizing_the_list

You can modify existing TextMate bundles (most can be found at https://github.com/textmate) and add them to Textastic: http://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html
Hi, you can try to use this markdown TextMate bundle with Textastic: https://github.com/fletcher/markdown.tmbundle

It seems like it does support highlighting of footnotes.

See http://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html for how to add it to Textastic.