As mbafk and josephturnip state, they simply put online the same copy that was published in the conference. Academic conferences typically publish papers in PDF form.
But, that doesn't actually answer your question, which I think is "Barely." I feel silly preparing PDFs for publication when I know that most people will read it on their computer, not print it out. Many conferences no longer even have an actual, physical copy of the proceedings, instead just giving out USB sticks with all of the PDFs. (Which is what we want anyway.)
I think it would be fantastic if there was a standard HTML5 template that researchers could use to publish their papers. There are Latex-to-HTML compilers, but I've never been impressed with the results. I think people outside of academia would be more likely to read our papers if they were in HTML rather and PDF.
I am outside academia but I read a lot of papers. For most people outside I suspect the biggest problem is paywalls and not format. If you don't read a paper because it is a PDF and not HTML then you weren't really interested. With chrome, it is not even an annoyance as when you had to load adobe.
One reason for PDFs is as you mentioned, latex to HTML results are typically poor. Diagrams are another difficulty that don't have an easy HTML solution. Other reasons I prefer PDFs are: though I never print, I often save papers to disk since I don't always find the paper when I go searching the second time (especially if it is months or even years after), there is a real benefit to being able to read a paper offline - I don't always have a connection to the net when I want to read and lastly, if you have an ereader such as the kindle, pdfs render well on them.
Paywalls are probably a bigger issue, but it's still friction. Anyway, modern browsers give the option to save all of the images along with the HTML file.
I agree it would be great if there was a good HTML template the IEEE or ACM used. Even using Latex you spend ages making it look exactly the way you want and don't want to upload a dodgy HTML version.
The major difficulty, I think, are figures. How can we maintain keeping figures near the text that talks about them, and also allows a high density of them? The best I can come up with is a narrow column of text on the left (similar in size to a column of text in an ACM or IEEE double column format), and a larger column on the right with figures. The difficulty is in anchoring the figures in the right column with the text. But then you may have a bunch of figures piled up in one place, and a sensible layout becomes difficult to do.
I guess they just uploaded the final version they published in EuroSys 2011. Like all ACM conferences I know about, PDF is the way it is done. They would have to reformat for HTML.
Mathematical formulas are a big reason; wikipedia uses images which look passable unless you start zooming in. Another reason is that with HTML you're never sure if you'll end up with the fonts as they were intended. Lastly, because HTML is not a page-oriented markup format, the presentation in general can differ from system to system; e.g., you can't say "on line 2 of page 3".
You may not have seen MathJax: http://www.mathjax.org/ The main drawback is that I noticed a delay before it fills in the correct math notation. But the results are quite impressive.
