TYPO3 Core

Why are you specifying a charset? Does your server deliver a wrong header for the file?

@Markus Klein: There are many reasons to use the @charset-declaration. But I think this is not the question, the declaration exists (http://bit.ly/1vjJCa2) and so TYPO3 should support it. Or not?!

Additionally:
If the charset is defined in more than one stylesheet and the concatenation is enabled, there are multiple charset-definitions.

1. the concatenation checks if the charset is on top. If not, move it to the top and remove duplicate definitions* and
2. the compressor adds the space between the charset and the charset-name (like the font-issue #60675).

*So we can provide the validity and fully support, if the concatenation is on or off.

In typo3/sysext/core/Classes/Resource/ResourceCompressor.php there is functionality to find and move @charset-etc. That's also where the moved-comment is inserted.

I have performed a test with both 6.2.14 and 7.4-dev (latest master);
1. I put in fileadmin the following files:

file1.css

@charset "UTF-8";
/*This is a mere test for comments*/
body{
  font-family:sans-serif;
}

file2.css

@charset "UTF-8";
/*This is a mere test for comments*/
body{
  font-size:14px;
}

2. TS Setup:

config.concatenateCss = 1
config.compressCss = 1
config.removeDefaultCss=1
page = PAGE
page.includeCSS {
  file1 = fileadmin/file1.css
  file2 = fileadmin/file2.css
}
page.10 < styles.content.get

3. Resulting file (typo3temp/compressor/merged-whatever.css?whatever):

TYPO3 6.2.14:

@charset"UTF-8";
@charset"UTF-8";
body{font-family:sans-serif}
body{font-size:14px}

TYPO3 7.4-dev:

@charset "UTF-8";@charset "UTF-8";body{font-family:sans-serif;}body{font-size:14px;}

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/47251

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/47251

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/47251

Patch set 1 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/47336