Kindle PaperWhite font checkboxes are inactive

Gents,

Concerning a Kindle PaperWhite, I have a font issue with one book.
I select any font checkbox (on the Kindle), there is no font change.
These options are working well with the other books.

I presume it's related to a weak HTML code on this specific book.
But what is the line to add or to remove in the editor ?

I appreciate any help, I don't know what to search for. I made some comparison, but it's tricky.

TIA