Running layout tests should not touch files in the tree, but this happens with imported w3c tests: 1$ $ git status On branch master Your branch is up-to-date with 'origin/master'. Changes not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: LayoutTests/imported/w3c/web-platform-tests/_certs/01.pem modified: LayoutTests/imported/w3c/web-platform-tests/_certs/02.pem modified: LayoutTests/imported/w3c/web-platform-tests/_certs/cacert.pem modified: LayoutTests/imported/w3c/web-platform-tests/_certs/cakey.pem modified: LayoutTests/imported/w3c/web-platform-tests/_certs/index.txt.old modified: LayoutTests/imported/w3c/web-platform-tests/_certs/localhost.key modified: LayoutTests/imported/w3c/web-platform-tests/_certs/localhost.pem modified: LayoutTests/imported/w3c/web-platform-tests/_certs/serial