Dots in tag names

[Helen Borrie]

Is it legal to use dots in tag names? 
For example,

part.title
part.number
chapter.title
chapter.number
body
body.first

and so on.

These tags are for chapters in a large book of several parts.  It is being written in unstructured Framemaker but eventually, after publication, much of the content will be converted to structured FM (DITA).  I am setting out to "fix" a template that has a lot of tags with spaces in them.  Already I have encountered some problems with spaces in tag names during trial conversions.  I want to avoid "camel case" tags if I can.

Is there any reason to avoid the dotted style?

Thanks,
Helen