Openlayers combat: loading KML files

In the interaction of OPenlayers, we often need to load some data. In this actual combat, the demonstration is to load KML files.
A file with the .KML file extension is a Keyhole Markup Language file. KML files use XML to express geographic annotations and visualizations by storing locations, image overlays, video links, and modeling information such as lines, shapes, 3D images, and points.

renderings

The demo.kml used

<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/km