From Xojo Documentation
Loads source (a String containing HTML) into the HTMLViewer. RelativeTo is a file (not a folder) used for resolving links and temporary files.
Any links will be resolved relative to the passed RelativeTo FolderItem.
On Windows, temporary files are created beside the passed RelativeTo FolderItem. If the temporary files cannot be created, then an exception error may be thrown. This can happen, for example, if you are trying to write to a read-only volume or you don't have write permissions.
In this example, the raw HTML is in a TextArea:
Loads the the supplied File into the HTMLViewer. File must be an actual file and not a folder.
This example loads an HTML file from disk. The file type has been added using the File Types Set Editor: