Intellij IDEA 2017.3 has got new project code style settings format. Now all code style settings are located under <PROJECT_ROOT>.idea/codeStyle directory with the ultimate intention to support multiple project code styles linked to different scopes. New code style settings format is also cleaner and contains all code style settings properly grouped per language. Some old legacy Java, HTML settings previously stored without any language tag are moved to "JavaCodeStyleSettings", "HTMLCodeStyleSettings" etc. respectively.
The old .idea/codeStyleSettings.xml is imported to two new files: .idea/codeStyles/codeStyleConfig.xml containing a link to preferred project code style and .idea/codeStyles/Project.xml with code style settings for different languages. The latter is optional and created only if there are non-default settings. If .idea directory is under version control, both files can be added to VCS too. The old codeStyleSettings.xml is kept for backwards compatibility.