I just tried to use the a:visited property in a CSS file to give links another color when the user clicked them.
It works fine in the browser, but it does not work at all in Notes. Don't know why.

But here is a workaround:

1.) define a CSS class of your own like this:

---
.clicked { color:red }
---

2.) write the HTML for the link like this:

---
a href="..." onclick="dojo.addClass(this, 'clicked')"...
---