Accent marks not easy to differentiate in this font
I'm having a hard time seeing the difference between the various accents in French in the font used. Even when I use the trick of control-+ to enlarge the font on the screen, the accents are quite small and not easy to distinguish. They are so short that the accent aigu and the accent grave look very similar. I know that at 46 my eyes aren't the best any more, but I can't be the only person who has trouble with this! :) I don't seem to have this issue in other French sites on the web.
I hope if you are considering any adjustments to the font you'll pick one that has clearer accent marks.
It's dark grey on light grey in the box where it shows the correct solution after you make an error. And the corrected words are bold which makes it even more difficult to distinguish.
Spanish only has forward accents so I've only had trouble with i and í. But I can imagine that it might be difficult to distinguish between á â and à in the blurry bold grey on grey.
I agree that the aigu and the grave are hard to distinguish. Although the tiny font size has something to do with it, at the same size the accents in Arial are pretty clear due to their off-centre placement. And Tahoma is even easier to read. I have zoomed in a little, but cannot zoom beyond 125% because Duolingo doesn't adapt properly.
Another problem that I have with the font is that when I zoom in I seem to be experiencing hinting issues. For example, the E in your post has a one pixel middle bar, but two pixel top and bottom bars. Another problem is features not lining up letter-to-letter which creates an uneven inconsistent look.
On a related note, I think Duolingo could be much more economical with screen space, which in turn would allow it to use a much bigger font. It is madness that we're even discussing this. How can we possibly have trouble fitting a few lines of text on a screen that is bigger than A4?