All the examples have two text paragraphs, one of which is large text. The first example has a white background and the second one has a black background. Sorry in advance for the eyesore.
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This example pair covers the requirements for 1.4.3 (Level AA)
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This example pair covers the requirements for 1.4.6 (Level AAA)
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Ensuring better contrast improves your site. If you have good color vision, you might not see anything too wrong with any of these. But just remember that some people do have problems with color vision or have a lower vision in general. Don't forget to include those users in your designs.