[Madin-H23]

Fixes #13.

cssutils creates the CSSUTILS logger at import time, adds a StreamHandler to it and forces the level to INFO. When an application configured logging before importing cssutils, that configuration was overridden, and a second ErrorHandler added a second handler.

This change:

• installs the default StreamHandler only when the logger has no handlers at all, so an application handler is left alone and no duplicate handler is added,
• applies the default level only when neither the CSSUTILS logger nor the root logger has an explicit level, so a level chosen by the application is preserved.

For an unconfigured logger the behaviour is unchanged (handler on stderr, level INFO).

One limit worth stating: the root logger's default level (WARNING) cannot be told apart from a level an application chose explicitly, so an application that sets exactly WARNING on the root logger and installs no handler of its own still gets the INFO default.

Verified locally:

• pytest: 629 passed, 43 xfailed
• ruff check and ruff format --check (ruff 0.17.1): clean
• mypy --config-file mypy.ini cssutils: no new findings (the 40 reported errors are pre-existing on main)

🤖 This contribution was drafted with AI assistance; the analysis, the change and the tests are described above so that they can be reviewed.

[Madin-H23]

The tests workflow on this fork PR is waiting in action_required (the first-time-contributor approval that fork PRs need). Could you approve it so the CI runs?

The change is small, and here is the local verification:

• pytest: 629 passed, 43 xfailed
• ruff check and ruff format --check (ruff 0.17.1): clean
• mypy --config-file mypy.ini cssutils: no new findings (the 40 reported errors are pre-existing on main)

The regression is reproducible on main alone: with the tests of this branch applied to the unpatched sources, test_keeps_existing_handler, test_keeps_configured_level, test_keeps_root_level and test_keeps_root_configuration all fail, because the logger setup forces the level to INFO and adds a second handler.