[dictum]

When it's about UI design, you know the advice is good when you see a lot of comments panning it on HN ;)

I just have two nitpicks: different font sizes are not a problem as long as the hierarchy is clear (i.e. sizes not seemingly chosen at random) and the advice on text contrast is accurate (unless you're using a display at very low brightness, pitch black text is jarring on light backgrounds), but the contrast on the example is too low. For body text, the ideal choice is closer to #222 or #333. Here, the advice on colors also applies: you can give the black a slight blue hue if the rest of the interface has blue accents.

The last example (https://cdn-images-1.medium.com/max/1600/1*cuYcwjOO26sKHImHa...) also moves me to give an unsolicited tip: pay attention to the "color" of the typeface you're using. That example has body text in Avenir (EDIT: looking again, it must be Proxima Nova); it's a thin geometric type, so the gray of the body text should be darker, or a bolder weight should be chosen. As it is, it's not very readable, which is okay for certain sentences but not appropriate for a warning message. Since system and webfonts are often constrained to regular and bold (and rarely, a bolder and a lighter weight) it's easier to control this by tweaking the gray.

An alternative is to give a higher priority to "you will lose all of your saved data".