All other elements start with an upper case letter, like RelativeLayout, TextView, etc.
Is there a particular reason why the Fragment element is always used as <fragment ... /> instead of <Fragment ... />?
Fragments when defined in XML aren't actually views. The full view name is specified, or it assumes android.view for the package and finds the rest. But a fragment is only truly specified when it includes the android:name tag. It acts like a keyword, which are typically written in lower case. Thus, Fragment refers to the class, and fragment is the placeholder in XML for fragments.
As Andrew mentioned in the comments, any special item, such as <include> and <merge> are written in lower case, as they are not android.view's.
来源：https://stackoverflow.com/questions/21948034/why-is-the-fragment-element-in-android-layout-written-with-a-lower-case-f