Colour contrast below the threshold - WCAG 1.4.3
why this one is a design decision, not a mechanical fix
Text needs enough contrast against its background to be read by someone with low vision or on a screen in daylight: 4.5 to 1 for body text, 3 to 1 for large text (roughly 18.5px, or 14px bold and above). Ace reports each instance it can measure.
Why no tool should fix this for you
Changing a colour changes the book's design. A pale grey caption might be the designer's deliberate hierarchy, or it might be a mistake nobody caught; only a person can say which, and only the publisher can approve a new palette. A tool that silently darkened your text would be making a design decision it has no standing to make.
The hand-fix
Find the rule in the CSS and darken the foreground or lighten the background until the ratio passes. Check the pair, not the colour: #767676 on white passes at 4.54 to 1, and the same grey on a cream page does not.
/* fails: 3.5:1 on white */
.caption { color: #949494; }
/* passes: 4.6:1 on white */
.caption { color: #757575; }A caution about dark mode
Many reading systems invert or re-colour text. Fixing contrast by hard-coding colours can make things worse in an inverted theme; where you can, let the reading system's own colours through instead of overriding them.
What Reflow does
Routes every instance to the specialist tier rather than touching your design, and names them on the report and in the certified summary so the claim stays honest about what remains open.