Your (linuxeventually) acronyms are flumoxed.

Video: VP8 vs H264 vs Theora
Audio: AAC vs MP3 vs Vorbis
Container: OGG vs Matroska vs MP4 vs 3GPP vs AVI

WebM is VP8+Vorbis+Matroska.

HTML5 OGG doesn't make a whole lot of sense. Both OGG and Matroska encapsulate a very large variety of video+audio codecs, with Matroska being easier to expand bugt OGG being more robust.