This directory contains testsuites for CSS WG specifications, including ones that do not strictly speaking define CSS features, e.g., Geometry Interfaces.
The directories should be named like the specification's shortname, but without any level suffix.
As the test harness relies on the largely undocumented old CSS build system, this directory has a number of test requirements specific to it:
support files for a given test must live in an adjacent support directory;
tests must have a <link rel=help> pointing to what they are testing;
for each spec so linked, test filenames must be unique; and
support and reference files must have unique filenames within the entire css directory.
vendor-imports/ is a legacy directory where third parties historically imported their tests that originate and are maintained in an external repo. Files in this directory should never be modified in this repo, but should go through the vendor's process to be imported here.
Given an old branch in git based against the old csswg-test repository, it can be moved over to the merged repo in one of two ways:
(Recommended:) Rebasing on top of web-platform-tests: with the old branch checked out, run git rebase -Xsubtree=css/ origin/master (or similar, depending on the name of the upstream remote).
Merging to web-platform-tests: with web-platform-tests' master branch checked out, run git merge -Xsubtree=css/ my_shiny_branch (or similar, depending on the name of your branch).
If you have a branch/bookmark in Mercurial, the process is more complicated:
From the Mercurial repo, run hg export --git -r 'outgoing()' > foo.patch. This will export all the changeset shown in hg log -r 'outgoing()'; it's recommended you check this is the right set of changesets before continuing!
Move to the git repo, and create a new branch based on web-platform-tests' master; e.g., git checkout -b hg-import origin/master (or similar, depending on the name of the upstream remote).
Download hg-patch-to-git-patch and run python2 hg-patch-to-git-patch < foo.patch > bar.patch (where foo.patch is the path to the foo.patch you exported above).
Run git am --directory=css/ < bar.patch.