Hotkeys to change appearance of links?

I am working on a site to help readers understand a book. To do this, I have the text of the book with an empty space to the side. Difficult words and phrases in the text are underlined links which, when moused over, make a note explaining the word/phrase appear in the empty space to the side of the text.

My problem: underline linking all those words is visually distracting, but the reader needs some way to know which words have notes attached to them so that they don’t end up having to run their mouse over every word. What I’d like to do is set up a hotkey that will change the appearance of the linked words:

hotkey off: text appears normal with no underlined links
hotkey on: text appears with words underline-linked

This way, readers can toggle the underlining/link color off and on as they need help with the text.

Any suggestions on how to do this? **Basically, I’d like to be able to toggle link appearance with a hotkey. **
Thanks for your help!