phpdocx 10 Premium licenses include the new option stylesReplacementType in HTML Extended, to keep existing placehoder pPr and rPr styles when being replaced by HTML.
Hello,
There's no method do it automatically. The easiest solution would be extracting the default information from the word/styles.xml file (w:docDefaults tag) and apply it to the HTML as a body (or another root tag) CSS.
The beta version of the Indexer class allows extracting that information (default font size, font family, lang and spacing); if you upgrade to the latest version of phpdocx we'll send you this new file and a script to illustrate how to that.
Regards.