The W3C defines an XML specification recommendation: http://www.w3.org/TR/REC-xml/

Since you've narrowed your question to XML formatting, there's no "universal" answer to how your XML should be formatted. Beyond conforming to whatever DTD or schema you happen to be dealing with, the importance of particular spacing/indentation of your tags lies with the people who will be dealing with your data.

If you're creating XML data to be sent across a network as part of a web service or some sort, then generally you're going to want to eliminate any unnecessary whitespace before transfer in order to optimize your data transfer rate. This means no line breaks, no indentation, no comments.

If your creating an XML document that others will be reading/modifying on a regular basis, then obviously you'll want to put some consideration into keeping the document readable. What constitutes "readable" is determined by everyone involved on that particular team or project.

• Intellij IDEA 2017.3 has got new project code style settings format. Now all code style settings are located under <PROJECT_ROOT>/.idea/codeStyles 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 into 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.

Original answer (now obsolete):

• .idea/codeStyleSettings.xml stores the project specific code style that is for sharing with the project so that all the team members have the same code style applied automatically for this specific project. Check this answer for details.

• .idea/codeStyles/codeStyleConfig.xml is used to override the default project code style. By default it contains <option name="USE_PER_PROJECT_SETTINGS" value="true" /> so that project specific configuration from .idea/codeStyleSettings.xml is used. If you switch your code style settings to use a scheme stored in the IDE instead of the project scheme, this file will store your choice.

• IntelliJ IDEA also has support for .editorconfig which will have the priority and override IDE and project scheme by default. Check the documentation for more details.

• codeStyle.xml in the project root is for Eclipse IDE and is not used by IntelliJ IDEA. It can be probably used by IntelliJ IDEA as well, if you have Eclipse Code Formatter plug-in installed and configured accordingly.

For your example with the <selector> in xml, it looks like the arrangement doesn't need to be changed, as the elements are already in the order you want them to be.

To change the formatting, go to Preferences --> Editor --> Code Style --> XML, and click on the "Android" tab. Change the "Wrap attributes" value (under the "Value Resource Files and Selectors" heading) from the default of "Do not wrap", to "Wrap always". Click OK.

Now, if you do a reformat via CMD + ALT + L or CTRL +ALT + L on the selector xml, it will produce the following:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape>
            <stroke android:width="2dp"
                    android:color="@color/colorPrimary"/>
            <corners android:radius="4dp"/>
            <solid android:color="@color/colorAccent"></solid>
            <gradient android:angle="45"
                      android:endColor="@color/colorAccent"
                      android:startColor="@color/colorPrimary"/>
        </shape>
    </item>
</selector>

It's not exactly as you desired, but at least the attributes have their own line, which aids readability.

If your XML is previously exported Intellij you can simply do:

File > Import Settings > select your xml

If its custom defined, one you can try copying your xml to intellij config directory:

config/codestyles

Assuming your xml is in format intellij can understand, it will then show up in

Code Styles > Manage

If your XML is in different format, try to match tags with one of pre-defined styles(also XML)