[roger64]

Thank you all for your comments.

Culprit found: line-height value

I checked the book which suffers from a faulty display of the value "text-top" with the help of the Inspector of the Calibre editor. I checked namely a sup tag.

I have found that at the above level (that is paragraph), the style definition used the value: line-height:140%;

Once removed this line-height value, the sup tag display was again correct. If one does not want to suppress this 140% value of line-height for paragraph, the most logic solution is to add a different value of line-height to the sup tag (as Jellby advised). This value will be displayed in priority due to the application of CSS rules.

Adding line-height:1; looks OK.

YMMV