DIVX videos, or rather videos compressed using the DIVX codec can be converter to several other formats including the WEBM format (Matroska).
This divx to webm conversion can be nowadays done with almost any decent multimedia converter, but keep in mind that regardless of the output format, the quality won't be better than the original. Checking out avi to webm conversion might be worthwhile, considering most DivX videos are in AVI container anyway.