Your comments

What does your html file look like?
You can add custom syntax definitions to Textastic by using TextMate bundles. See
A quick search for "Rust tmbundle" in Google brought up these TextMate bundle which might work:
Please try to enable the "Debug Log" setting in the connection's settings screen. Tap on the (i) button on the right side of a connection to change its settings. This creates a log file when you try to connect to the server which might help you find out what's wrong.
Hello,

1.) yes, you can just create a new file with a .css file extension
2.) you can add a link to your CSS file like this:

<link rel="stylesheet" href="example.css" type="text/css" />
Textastic is not scriptable at the moment.

But, if you add the AppleScript Editor to System Preferences -> Security & Privacy -> Privacy -> Accessibility, you can use the following commands:

get:

tell application "System Events" to tell application process "Textastic"
get size of window 1
end tell

set:

tell application "System Events" to tell application process "Textastic"
tell window 1
set {size, position} to {{1280, 800}, {50, 50}}
end tell
end tell

Unfortunately I can't reproduce this issue on my iPad.

Can you maybe send me a screenshot or small video demonstrating the issue?
For the moment, you can add this syntax definition yourself.

This is explained in the manual: http://www.textasticapp.com/mac/manual/lessons/How...

The Arduino bundle can be found here: https://github.com/nasser/arduino.tmbundle

I will add Arduino support in the next update of the Mac version.
Currently, you can tap "Done" and tap on the preview button again to refresh.