@justsomeone1 That was useful, it gave me the idea of testing with different profiles and browsers but it didn´t work in any of them. Finally, looking at it closely, I discovered where the problem was! No more scratching my head!
I wasn´t really accurate when I said I had saved the HTML file to a USB thumbdrive, what I had done is to email the HTML file to myself first, and looking at the HTML source code I can see that my email provider has embedded its own stylesheet in the HTML file which broke the structure when I imported it.
The way to fix it is by not downloading the attachment using the “Save link as…”, but using the proper email download, this way it comes right like you uploaded with no modifications.