Editing text in page view does not work properly when using p:first-letter { font-size: 125%; }

My CSS file contains a rule to display the fist letter of a paragraph at 125% size.

p:first-letter {
	font-size: 125%;

This causes strange behaviour:

  1. On page view, I select a paragraph, ant the blue border and menu appear,
  2. I click the “Edit text” menu icon and position the insertion mark in the text (here: in front of the word “Termin”, see screenshot).
  3. I type “x”; the “x” does not appear in the position of the insertion mark but a few characters further left.
  4. I position the insertion mark to the right of the misplaced “x” and press backspace to delete the “x”, but instead the whole paragraph text is gone.
  5. I press CMD-Z for undo and the text is restored twice.

See http://detbert.de/ws/mitglieder/aktuelles.html