The exact WCAG 2.1 contrast ratio for a foreground/background pair, with pass/fail for every threshold: AA and AAA on normal text, AA and AAA on large text, and the 3:1 line for UI components and focus indicators.
The luminance math is the spec’s own (sRGB linearization, then the 0.2126/0.7152/0.0722 weighting), so the number matches what auditors compute.
Frequently asked questions
What counts as large text?
18.66px bold or 24px regular and above — those only need 3:1 for AA.
Does 4.5:1 guarantee readability?
It guarantees distinguishability for normal vision. Low-vision users benefit from more; aim for AAA (7:1) on body copy when you can.
中文说明
给出前景/背景组合的精确 WCAG 2.1 对比度，并对每条阈值逐一判定：正文与大字号的 AA 和 AAA，以及界面组件与焦点指示的 3:1 线。
亮度算法用的就是规范本身（sRGB 线性化后按 0.2126/0.7152/0.0722 加权），数字与审计工具一致。
常见问题
多大算大字号？
粗体 18.66px 或常规 24px 及以上——这些只需 3:1 即可过 AA。
4.5:1 就一定可读吗？
它保证正常视力的可分辨。低视力用户需要更高；正文尽量做到 AAA（7:1）。