Your comments

No, it's not supported in the current version of Textastic.

I plan to make it possible to add custom syntax definitions in Textastic 3.0.
csv

What exactly is missing for you in order to work with CSV files?

Do you mean that it should be possible to preview .css files that are not used in an html file? That doesn't make much sense because css stylesheets are used to style html files and don't have much meaning by themselves.

You need to reference a css file with the proper html tags in an html file and preview the html file instead.
Of course, a CSS file that is in the same directory as an HTML can be used directly like that:

<link href="screen.css" rel="stylesheet" type="text/css" />

Also, inline CSS is supported. I use it all the time. In fact, everything that's supported by Mobile Safari should also work in Textastic's preview. What exactly did you try?
Does this happen even when you just have a plain text file with only a few lines or does it happen with a specific syntax highlighter and large files?

To add to the point about the WebDAV server – I just remembered why I enabled this setting in the first place (I implemented this about 9 months ago):

It's not possible to start the WebDAV without a Wifi connection. The WebDAV configuration screen in Textastic detects if a wifi connection is available and shows an error message if it is not. Without a wifi connection the iPad wouldn't have an ip address assigned in the local network and so the server couldn't bind to an ip address. Without the UIRequiresPersistentWiFi setting, iOS wouldn't connect to the network by itself and so it would never be possible to start the WebDAV server.

Thanks for the detailed description. I'm pretty sure that this is caused by the setting I described.

iOS tries to make sure that you are connected to a Wifi network when Textastic is started because the app basically says that it requires a persistent Wifi connection. So, it does show the Starbucks login screen so that you are properly connected.

But, it doesn't send any data, it just connects to the Wifi network so that connections to remote servers work immediately when the user wants to work with remote files and also as I mentioned to make the builtin WebDAV server work reliably.
Hello,

Textastic does not try to send data to a server when you launch it. There is no code that connects to any server without the user initiating such an action. Also, there isn't any code in Textastic that "phones home" nor is there any analytics code in the app.

Why do you think it would do that? Can you give me more details? Did you get any messages or alerts saying otherwise? Did your Firewall intercept anything?

The only thing I could see is a setting in Info.plist: the setting UIRequiresPersistentWiFi/"Application uses Wi-Fi" is set to "YES". The description for this setting in Apple's documentation is: "specifies whether the application requires a Wi-Fi connection. iOS maintains the active Wi-Fi connection open while the application is running."

This is enabled so that the builtin WebDAV server works without problems. I think iOS also prompts the user to connect to a Wifi network when this is set. Maybe this is what you are seeing?
Thanks, I think I know what's going wrong and will try to fix this for the next version.