Learn Accessibility by Building a Quiz - Step 46

I do not know what a contrast ration of at least 7:1 means or how I can find out what color would be contrasted at that ratio. The hint gives me the answer but I don’t know why it is correct. Could someone explain what contrast ratio is and how to figure it out? (I know ‘’‘color: blue’‘’ is the incorrect answer)

li > a {
color: blue
}

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Challenge: Learn Accessibility by Building a Quiz - Step 46

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