XML file encoding format "utf-8" VS "UTF-8"?
The IANA character set registry says:
no distinction is made between use of upper and lower case letters.
But that page, the XML specification, and unicode.org are consistent about capitalizing UTF-8.
From the XML specification:
"XML processors SHOULD match character encoding names in a case-insensitive way"
This indicates that you can use upper case or lower case or even mixed case if you wish. However, the specification uses "UTF-8" in all its examples so for consistency I'd go with that.
For those interested in the gory details - including links to some of the related standards and precedents - I blogged a couple of years ago about Case-Sensitivity of UTF-8 in XML Declarations.