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Chrome and Gecko seem to use the upper case version, IE might use lower case though.
Charset/encoding labels are case-insensitive all over the Internet, so with respect to functionality, this shouldn't matter at all. And the spec explicitly says so, please look for http://encoding.spec.whatwg.org/#ascii-case-insensitive. If it makes a difference, then something somewhere (browser or server or other) is broken, and should be fixed. If this is indeed the case, it would be good to have examples. As for whether the spec should use 'utf-8' or 'UTF-8', I personally would prefer 'UTF-8', because being an acronym, that's how it's written in books and other documentation. But looking at the spec, that would mean that many other labels (starting with US-ASCII and ISO-8859-1) would have to be upper-cased, too. As this is a purely editorial issue, we can leave that to the editor. An alternative is to provide a pull request. What I'd like to avoid is to change the case of the file names for the data files for the encodings. Software that handles that may not do case-insensitive there. (But if we change these too, that's not the end of the world either.)
This is not an editorial issue; it affects what document.characterSet returns.
IE10 uses lowercase. I think last I checked Gecko used lowercase for gb18030 or some such. Gecko was not consistent and neither was Chrome. All lowercase seemed better therefore as it's easier to predict.
*** This bug has been marked as a duplicate of bug 19395 ***