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Hello, you can add custom syntax definitions to Textastic by using TextMate bundles. See
A quick search for "GLSL tmbundle" in Google brought up this TextMate bundle which should work:
HTML 5 animations should already work.
I assume this is a problem with the AZERTY keyboard layout. Which character is inserted when you press Alt-I?
Hmm, do the other shortcuts like Cmd-Alt-P (Preview) work?

Does it work after you quit and restart the app?
Unfortunately that's a limitation of iOS. The new iOS 7 keyboard shortcut API doesn't autorepeat pressed keys.
This is going to be the new app icon in Textastic 5.0 which is currently waiting for review and should be available on the App Store soon:

Here are some screenshots of the new version of Textastic for iPad: