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This resource will explain a bit further: www.w3.org
"ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".")."
Please Note that even though W3 provides a solution, this does not mean that it is compatible with all web browsers or that it is current information.
"ID and NAME tokens must begin with a letter ([A-Za-z]) and may be followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), and periods (".")."
Please Note that even though W3 provides a solution, this does not mean that it is compatible with all web browsers or that it is current information.