The office suite Calligra can save many file formats, but the main one is OpenDocument Format. With a proposed improvement, it can be easier than ever to code on Calligra.
The OpenDocument files (odt, ods, odp) that Calligra writes are zip files with XML documents and images. The XML documents have a schema, a set of rules that say what goes where in the files. Here is a small sample of ODF, indented for readability: