WebM is an open video format for the web.
As of writing, only the Chrome browser supports Opus audio in WebM container, but expect VLC Media Player and other browsers to support it soon. Get Google Chrome here.
In case your device runs Android 4.1+ and comes with licensed H.264 and AAC encoders, IP Webcam can also record video into widespread MPEG4 format. Note that there might be restrictions for recording and distributing the resulting video in this format.
Select the desired format in IP Webcam Settings menu ⇨ Video Preferences ⇨ Video Recording ⇨ Video Format.