I'm not sure if this is the right thread for my question.

Anyway, why is it that RadSpell control doesn't display a "red line" below those mispelled words ?

Hi,

The dialog based standalone RadSpell underscore the misspelled works with red dots (see screenshot). The inline spellchecker of RadEditor highlights the misspelled words with yellow background (see screenshot). If you want to change the yellow background with red underline wave see this KB article on the subject: Highlighting the misspelled words with red wave underscore (AjaxSpellChecker Only).

The typos are highlighted with a different color in the editor, because the built-in browser's spellchecker uses red wave to underline the misspelled words.

All the best,
Rumen
the Telerik team