By default, if a webpage doesn't specify a background, 3D WebView sets its background to white in order to mimic the behavior of normal browsers. However, a web page can opt into being transparent by including the following meta tag in its head:
<meta name="transparent" content="true">
For example, 3D WebView's keyboard UI is a React.js app that uses that tag in order to have a transparent background. All 3D WebView packages support transparent webviews except for 3D WebView for Android with Gecko Engine, which is currently unable to support transparency because Gecko's compositor lacks that capability.