When you try to add your own functions for generating a specific css code that should to be put inline, ezjscore outputs not valid xhtml acording to the w3c validator.
Problem come because of ezjscPacker::buildStylesheetTag
When is not a css file but a css content, function output
but rel attribute is not supported by style tag, according to xhtml 1 transitional DTD .
I think we can safely remove that rel=\"$rel\" for this case.
Attached is a small extension to ilustrate the problem. i've called it 'test'. unpack the files in your extension folder and activate the extension.
regenerate your autoloads and your caches
give permissions to anonymous users to ezjscore/call/test_testcss.
if all goes as expected and you're using a standard you will see color of main text will turn red. but if you try to validate againts w3c validator, you will get errors related to the style tag produced.