Your comments

It works when the XML file has the correct styling tags and the CSS file is written correctly.
In Textastic 2.1 you can scroll the additional row of keyboards horizontally. If you move the row to the left you can get to the semicolon and other keys.
Unfortunately, I haven't received the files yet. Update: got it now and will report back :)
Actually, I just tried to reproduce the problem. For me, it works like it should when I try to preview an XHTML file with a .xml file extension – the referenced CSS file is interpreted correctly.

Please make sure that your file is a proper XML file and contains no errors: try to run your file through the W3C validator at http://validator.w3.org/ and make sure to choose an XHTML doctype.
Hi, I changed the behaviour of the web preview in Textastic 2.1.

The file interpretation depends on the file extension. Previously, XML and SVG files where interpreted as HTML files which caused them to render wrongly. Also, mixed namespaces for xml files didn't work before.

Can you please send me the file(s) that cause a problem for you to support@textasticapp.com so I can see if I can reproduce the problem?

You could also try to rename your file to have an "xhtml" or "html" extension and I'm quite sure it will show the css correctly.
Since I had to write my own text selection routines for syntax highlighting to work, this would be possible.
Hi Nicola,

please enable the "Debug Log" setting for your FTP connection. Then try to download the file and send me a copy of the log file which should be in the "logs" directory to support@textasticapp.com
Done. Those will be added in the next update.
Yes, as Eric pointed out, Textastic has potentially sensitive data: some people want to protect the files they have saved in Textastic - the files may contain passwords for mysql databases or code that is under an NDA – other people need to protect the remote connections so that their kids can't delete files from their servers instead of playing games on the family iPad :)

I, too, don't want to protect the whole iPad with a PIN lock but being able to protect sensitive data on an app-by-app basis would be nice.