A WebM file is a video saved in the .WEBM format, sometimes with video alpha transparency. WEBM files are supported by the EngagePHD software, but only LG webOS and Windows devices will support playback.
The example below shows two WEBM files of a rotating gear and mascot in the Layout Designer. These videos have transparency so content (background) can be seen behind the objects in the WEBM video file.
Please also see: Best Practices for Asset File Sizes and Formats