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In terms of your question, I think HTML5 may be more limited when loading resources manually as you don't get access to the file system. I haven't looked into it that deep, but that may be part of the problem (for example, if you are loading a file manually). If the file is a texture or image imported into Godot, you may have to mess with the import settings. For example switching it to an image or a texture depending on what you are doing.
