Free tools
Two tools you can use right now, no account needed. They run in your browser and send nothing anywhere.
Contrast checker and fixer
Two colours in, ratio out. See AA and AAA results for normal text, large text and UI components, get the nearest passing colours that keep your hue, and preview the pair as people with colour-vision deficiency see it.
Open the checkerType scale checker
Set a base size, a ratio, a line height and a line length. Watch it live, check it against readability advice and copy a fluid clamp() type scale as CSS.
Part of the full suite
These tools are small versions of two A11yMate modules. Colour Studio and Typography Studio do the same thing for every colour pair and every type size found on your WordPress site.
Looking for something else? The rules reference is free to read, too.