By default all XML documents use Unicode characterset (utf-8 encoding).
If you have not explicitly used software that encodes characters into Unicode, or if your document uses characters other than "us-ascii", that is, a-z, A-Z, 0-9, then you need to explicitly define which characterset you are using.
These are the two rules for XHTML 1.0:
<meta http-equiv ... /> element is used.Both #1 and #2 should be added to the document. (In fact, only if the characterset encoding is other than utf-8, #2 is required, but it is good idea to add both anyway.)