When 'IJ' appear together at the beginning of a paragraph (eg. in "IJzertoren is een toren en vredesmonument in de West-Vlaamse stad Diksmuide."), both the first two letters should be highlighted.
The CSS specification specifically mentions that the IJ combination should be treated as a single letter.
Tests & results:
Interactive test Dutch IJ are selected as a single grapheme by ::first-letter
Even with the language of the text set to Dutch, neither Blink nor Webkit select IJ together.
This has been fixed, however, for Gecko. (Note that it only works if the language of the content is set to Dutch.)