The macOS version and the iOS/iPadOS version of Textastic are two separate apps for two different platforms with different feature sets.

Notable, the macOS version doesn't have remote connection support and does not have a built-in web preview.

If you are not happy with the app, you can request a refund from Apple on the following website: https://reportaproblem.apple.com

Yeah, with Split View this is definitely possible on iPad.

Thank you, I appreciate it!

I see. Yeah, please only add the parent folder as an external folder.

Textastic doesn't know the relationship between external folders. 

If you use the "Add External Folder…" command, the system presents the Files app user interface in a secure way (using XPC). 

When the user selects a folder, Textastic gets a so-called "security scoped URL" from the system. Opening this URL extends the app's sandbox so it can now access that folder.

Textastic has very limited information about folders opened in this way. For example, it does not know that folder "ChartUse" and "ChartClass" share the same parent folder.

That's why relative URLs only work within the same location (Local Files, iCloud, or an external folder).

Please also have a look at this topic in the manual: https://www.textasticapp.com/v10/manual/managing_files/local_files_icloud.html

It tries to explain the Local Files and iCloud locations as well as external folders.

Click the "iCloud" location that is shown in this screenshot and create a file or folder in it using the "+" button. Then you should get a new folder called "Textastic" in the Files app that has Textastic's app icon on it.

I'll try to explain:

Each app can have a folder in iCloud Drive that it can access without the user needing to add it manually. 

This folder only appears in the Files app when there is at least a single file in it.

So, please go to "iCloud" in Textastic and add a file or folder.

Then, a folder called "Textastic" should appear in the Files app at iCloud Drive/Textastic with Textastic's app icon on it.

This is the recommended approach.

Alternatively, you can give Textastic access to any folder in iCloud Drive by using the "Add External Folder…" command. This also works for folders in "On my iPad" or with folders in the Git client "Working Copy" among other things.

So, if you have a folder in iCloud Drive called "Projects" which in turn contains "ChartClass" and "ChartUse", add the "Projects" folder as an external folder to Textastic and it should work.

What I just noticed: in my screenshot, the back button is labelled "iCloud" while in your screenshot, the back button is labelled "Textastic".

Have you maybe added two folders in iCloud Drive using "Add External Folder…" and are trying to reference from one external folder to the other?

This won't work. It needs to be the same external folder. So you need to add the parent folder that contains the two subfolders that contain your files.

I unzipped your two .zip files into Textastic's "iCloud" location.

Both "relative.html" and "relativeTest.html" work for me.

Here are screenshots from my iPad: