XML Naming Conventions : Well Formed « Introduction « XML Tutorial

Names can start with letters including non-Latin characters.
Names can start with dash (-) character.
Names cannot start with numbers or other punctuation characters. 
After the first character, numbers, hyphens, and periods are allowed. 
Names can't contain spaces. 
Names can't contain the colon (:) character. 
Names can't start with the letters xml, in uppercase, lowercase, or mixed.
There can't be a space after the opening <
There can be space before the closing > character. 

Here are some examples of valid names: <first.name> 

Following are some examples of invalid names: 

<xml-element> which starts with xml, 

<123> which starts with a number, 

<your=xml> because the equals sign (=)sign is illegal, and 

<your element> which contains a space.

1.3.Well Formed
1.3.1.XML documents must adhere to certain rules to be well formed.
1.3.2.XML Naming Conventions
1.3.3.Case Sensitivity
1.3.4.Writing Non-Empty Elements
1.3.5.Nesting Elements
1.3.6.Empty Elements