Here are my two thoughts for the challenges I worked on today:
Contrasting Colors
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Interestingly, the color choice used in the Adjust the Tone of a Color challenge fail to pass the WebAIM color contrast test. This means that some people trying to pass this challenge may have difficulty determining the difference between the font color and the background color. I strongly recommend using the WAVE tool on your projects before submitting them in order to resolve color contrast issues like this and other accessibility-related issues…
CSS Animations
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I LOVE that CSS animations are included so early on in the curriculum! I think it really gives a sense that someone just starting out can have a lot of power over the design experience. It also shows that the complex animations one sees on polished sites isn’t as difficult to create as they seem. I don’t think I’ll come away from these challenges having mastered CSS animations, but it’s a great intro! 
I think this is soooo much better than suggesting new learners learn Bootstrap and jQuery! I especially like the Make a CSS Heartbeat challenge.
I’ll need to review the fCC challenges a few more times, build stuff with animations, and make use of the resources below before I’ve mastered CSS animations. This is a nice start though. 