Root.xsd used to validate schemas:
http://sdformat.org/schemas/root.xsd
Here is a list of some PX4 SDFormat files:
- Typhoon H480 drone - https://github.com/PX4/PX4-SITL_gazebo/blob/master/models/typhoon_h480_ctrlalloc/typhoon_h480_ctrlalloc.sdf
- Typhoon H480 World - https://github.com/PX4/PX4-SITL_gazebo/blob/master/worlds/typhoon_h480.world
A (could-be-)useful validation script from PX4:
https://github.com/PX4/PX4-SITL_gazebo/blob/master/scripts/validate_sdf.bash