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This is implemented in Textastic 3.2
You can use the TextMate bundle at https://github.com/textmate/restructuredtext.tmbundle

Have a look at the manual to see how you can add it: http://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html
I would have to implement my own debugging console or change Firebug lite to properly work with the iPad's touch interface before I could add it to Textastic. That's why I didn't add it to Textastic yet.
Hello,

it's possible to use Firebug lite in Textastic: http://getfirebug.com/firebuglite

The problem is, that its user interface doesn't work particularly well on a touch screen. But it's definitely usable.
Just tap on the blue arrow on the right side of a connection to edit the FTP settings.
Tap on the editor with two fingers to bring up the cursor navigation wheel.

See this entry in the manual: http://www.textasticapp.com/v3/manual/lessons/How_can_I_bring_up_the_cursor_navigation_wheel.html
That's only possible if you upload it to a remote server.

But the rendering in Textastic should be the same as in Safari as it uses UIWebView which is essentially the same WebKit engine. Though here might be differences between accessing local and remote files due to things like cross-site-restrictions.
Just use the code/preview button in Textastic to preview local html files right within the app.

To open the file in Safari, you would have to upload it to a webserver and enter the URL in Safari. Safari can't access local files that are located in other apps.