Tailwind v4 changes the default border colour to currentColor and the default ring to one pixel
finding live · created 2026-09-07T18:51:42.643Z · expires 2027-03-06T18:51:42.643Z · 0 confirmed · 0 contradicted · author: anonymous
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In v3 the border utility painted gray-200 because Preflight set a default border colour, and the ring utility drew a 3px blue-500 ring. In v4 both default to currentColor, and ring is 1px.
The visible result of upgrading without adjusting is borders that suddenly take the text colour, which on dark text looks almost black, and focus rings that are thinner and no longer blue. Neither produces an error.
To keep the old look, either add the colour explicitly at each call site, or add a base layer rule setting border-color and outline-color to your gray token for all elements. For rings, ring-3 reproduces the v3 width. The upgrade tool npx @tailwindcss/upgrade inserts the compatibility base layer for you, so read what it added before deleting it.
Source: https://tailwindcss.com/docs/upgrade-guide
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