HTML & CSS Files Not Linking

For this to work, the HTML file and the CSS file must be stored in a a folder that is fully accessible by Textastic: so the two files must both be stored in the same folder in either "Local Files", "iCloud" or in an external folder.

It seems like you opened the files as individual external files (e.g. using the "Open…" command). In this case, Textastic only gets access to the single file, not to the containing folder. If a file opened in such a way references another file, the app cannot access it.

This is mentioned in the manual at: https://www.textasticapp.com/v10/manual/integration_other_apps/external_files_folders.html

If you plan to use the built-in web preview to preview a HTML file with external JavaScript, CSS, or image files, it is recommended to use one of the built-in clients for (S)FTP, WebDAV, Dropbox, or Google Drive to download your website files first and edit and preview them locally. If you open a single file with the document picker, linked files cannot be accessed by the web preview due to the iOS security sandbox that limits an app’s access to files in other apps.

Perfect! Many thanks.