Widget runtimes are W3C Widget enabled runtimes. Those apps are sometimes also called HTML5 Apps. But they all have a W3C Widget inside. Unfortunately this spece is really crowded and hard to oversee, the following tries to give you an overview of the existing runtimes. Feel free to contribute to the overview and information.
The pure W3C widget runtimes are those that "only" provide HTML5 packages
WAC was announced in February at MWC 2010 in Barcelona. The specification version 1.0 is basically JIL 1.2.2 with a reduced feature set. Due to the low quality of the WAC 1.0 specification and the missing implementation details the WAC worked hard on version 2.0. Which is due to be announced in
Find more info about the MWC release at Widget runtime: WAC-1.0-compliant Golden for Android Or directly the WAC 1.0ÑSupporting runtime for Android: Installation instructions and release notes, Feb 14, 2011