This could be a very useful tool if it wasn't so erratic. It is very hard to use. Highlight a word and the menu doesn't always come up or if it does, either the menu disappears before you can click on it or the highlighted text loses its highlight Nor when a word ts highlighted, does it highlight every instant of the word. If a mistake is made and you try to delete it, it removes all highlighting as well as the mistake and then the extension will not work at all. It would be helpful if there was a drp down menu on the right click. All in all, it is a very frustrating tool.
One serious problem is that if a one or two syllable word is highlighted and the same syllables appear in three or more syllable words, the one or two syllables are highlighted in words with more syllables. Example: highlight the word "sane" and the letters will be also highlighted in the word "insane."
Another problem is that if by mistake not every letter is selected in a word that is highlighted and the user later tries to correct this by selecting either the entire word or the missing letter or letters and highlight them, HIGHLIGHT ALL will highlight the missing letter(s) and split the word into two parts while highlighting all of them.
In response to an earlier comment herein about the difficulty of deleting an erroneous highlight without removing all the highlighting in the document, the app author suggested clicking on the erroneous highlight and deleting it. It doesn't work and remains a serious defect.