I do know this isn't a bug or a problem with Ubuntu as such, but I couldn't think of a short subject that encapsulated my problem any better. And I'm probably missing something obvious, but I have tried to figure this out myself.
To set up the background, I have a set of HTML files which use a custom CSS file. These files are not online, they are for my own use, on my own machines or a USB stick. They contain most of the writing I've done (and will contain the rest when I get the time to add the rest of the files). On Windows XP, under Firefox, they work fine. (The last I checked, the formatting was broken in Opera, but that's presumably because each browser implements the HTML codes differently.) On my PortableApps USB sticks, they work fine. So the relative paths seem to be correct.
Now, I've also copied exactly the same files over to a ThinkPad running Ubuntu. And, although the HTML is there, and the links work, the CSS has no effect at all, in any browser. So I did what I could to track down what was going wrong. I installed Firebug, and it shows the the line calling the CSS file is working - but the CSS file appears totally empty in Firebug as if it contains no style info at all. So I assumed I might have saved it in a text format Linux doesn't like, so I opened up the file in a text editor in Ubuntu, then resaved it. The problem persists... At this point, I'm going crazy.
As I said, I'm probably missing the blatantly obvious. But if any of you can guess what is giving me so much trouble here, with what is meant to be a simple, universal set of files, I'd appreciate some advice. Specifically, if at all possible, I want to be able to use these files on a CD or a USB stick in any OS, without having to tweak them for each one. The fact they don't work perfectly in every browser isn't such a big issue, since Firefox is common enough for my purposes. But these are supposed to be an emergency backup set of all my manuscripts, and the fact I can't make them work as intended is driving me insane.