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Cool, thanks, that looks really promising!
Looks like there's enough that can be done in the next few years :)

I'm currently considering writing a new, advanced syntax highlighter that would allow me to do stuff like that.
I have to admit that I've never tested it over VPN and unfortunately I don't have a working VPN server... I'll try to reproduce it!
Textastic 2.0 is now available on the App Store!

New Features:
• FTP and Dropbox client
• Find and replace
• Rename, copy and move files and folders
• Web preview for Markdown files
• Send HTML and Markdown files as HTML e-mail
• Support for multitasking
• Select a line by tapping on the line number
• Read-only mode
• Settings for auto-correction and auto-capitalization
• Added Inconsolata font

Fixed:
• Lines are now broken on word boundaries
• Chinese and Japanese characters are now displayed
• UTF-16 files can now be opened

Thanks to your feedback, this version also fixes a lot of other minor issues.
Danke! Textastic 2.0 ist mit FTP und Dropbox-Unterstützung ist soeben im App Store erschienen! :)
What's not working with XHTML for you? Just the preview (this should work in Textastic 2.0) or do you have problems with the syntax highlighting?

The "HTML" syntax highlighting definition should work for XHTML as well.

If you have problems with a particular file, please send the file to support (at) textasticapp.com so I can reproduce it.
Oh, and you can select XML syntax highlighting for SVG files by manually choosing it in the "File Information" popover for now.
If it doesn't work for you in Textastic 1.1 then it's probably the same issue as with php files: see http://feedback.textasticapp.com/feedback/8060-web-preview-of-php-files-doesnt-interpret-html-part/ – I've fixed this in Textastic 2.0 which is currently in review at Apple.

I'll add the svg extension to the XML syntax definition for a future version.