The specification is massive (840 pages) even though it is written in very terse way that does not really specify the effects and behavior, only the syntax.
On the other hand if one ignores few warts (explosion of local styles and text spans due to ooo:rsid attribute, non-sparse spreedsheets and weird mechanism for styling tables as a few examples) it is really well designed markup for this kind of document data that strikes right balance between it being semantic markup and representing the kinds of stuff users want to do. Compare that with Office OpenXML with stateful formatting empty tags (yes, really, in DOCX <b/> _TOGGLES_ whether following text is bold).
On the other hand if one ignores few warts (explosion of local styles and text spans due to ooo:rsid attribute, non-sparse spreedsheets and weird mechanism for styling tables as a few examples) it is really well designed markup for this kind of document data that strikes right balance between it being semantic markup and representing the kinds of stuff users want to do. Compare that with Office OpenXML with stateful formatting empty tags (yes, really, in DOCX <b/> _TOGGLES_ whether following text is bold).