Describe the bug

When base: './' is set and the build is configured to emit CSS into a subfolder (via assetFileNames) while keeping images in a sibling folder,
Vite 8 writes url(./img/...) into the CSS instead of url(../img/...), which Vite 7 writes.

Reproduction

https://github.com/karlvr/vite-relative-base-bug

Steps to reproduce

npm install
npm run build:v8

Inspect ./dist/css/main-*.css and observe that the url to the background image is incorrect. It should be ../img/foo-*.png but it's ./img/foo-*.png.

You can compare with Vite 7 by running:

npm run build:v7

System Info

System:
    OS: macOS 15.7.5
    CPU: (10) arm64 Apple M1 Max
    Memory: 119.17 MB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.15.0 - /Users/karlvr/.local/share/mise/installs/node/24/bin/node
    npm: 11.12.1 - /Users/karlvr/.local/share/mise/installs/node/24/bin/npm
    pnpm: 11.0.8 - /Users/karlvr/.local/share/mise/installs/node/24/bin/pnpm
    Watchman: 2026.05.04.00 - /opt/homebrew/bin/watchman
  Browsers:
    Chrome: 147.0.7727.138
    Firefox: 59.0.3
    Safari: 26.4

Used Package Manager

npm

Logs

No response

Validations

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• Read the Contributing Guidelines.
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• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.