Currently there are two different flags which pull the same media-libs/libvpx for the same purpose.
vpx : Enables vp8 codec support using libvpx, required to play some HTML5 videos
webm: Use system media-libs/libvpx for HTML5 WebM video support.
Generally webm is somewhat wider term than vp8 (see e.g. here: http://en.wikipedia.org/wiki/WebM), but in all ebuilds I have checked webm USE flag just pulls the vp8 implementation library - libvpx.
I am not 100% sure, but to me it seems like webm and vpx USE flags should be unified.
$ equery hasuse -p vpx
* Searching for USE flag vpx ...
[-P-] [ ] media-plugins/gst-plugins-meta-0.10-r6:0.10
[IP-] [ ] media-plugins/gst-plugins-meta-0.10-r7:0.10
[-P-] [ ] media-video/avidemux-2.5.6-r1:2
[-P-] [ ] media-video/ffmpeg-0.7.8:0
[-P-] [ ] media-video/ffmpeg-0.10:0
[IP-] [ ] media-video/ffmpeg-0.10.2:0
[-P-] [ -] media-video/ffmpeg-9999:0
[-P-] [ ] media-video/libav-0.6.5:0
[-P-] [ ] media-video/libav-0.7.4:0
[-P-] [ -] media-video/libav-0.7.9999:0
[-P-] [ ] media-video/libav-0.8:0
[-P-] [ ] media-video/libav-0.8.1:0
[-P-] [ -] media-video/libav-0.8.9999:0
[-P-] [ -] media-video/libav-9999:0
[-P-] [ ] media-video/mplayer-1.0_rc4_p20110322-r1:0
$ equery hasuse -p webm
* Searching for USE flag webm ...
[-P-] [ ] mail-client/thunderbird-8.0-r1:0
[-P-] [ ] mail-client/thunderbird-9.0:0
[-P-] [ ] mail-client/thunderbird-10.0:0
[-P-] [ ] mail-client/thunderbird-10.0.1:0
[-P-] [ ] mail-client/thunderbird-10.0.1-r1:0
[-P-] [ ] mail-client/thunderbird-10.0.3:0
[-P-] [ ] mail-client/thunderbird-11.0.1:0
[-P-] [ ] net-libs/xulrunner-2.0.1-r1:1.9
[-P-] [ ] www-client/firefox-8.0:0
[-P-] [ ] www-client/firefox-9.0:0
[-P-] [ ] www-client/firefox-10.0:0
[-P-] [ ] www-client/firefox-10.0.1:0
[-P-] [ ] www-client/firefox-10.0.1-r1:0
[-P-] [ ] www-client/firefox-10.0.3:0
[-P-] [ ] www-client/firefox-11.0-r1:0
[-P-] [M ] www-client/icecat-10.0-r1:0
[-P-] [M ] www-client/icecat-10.0-r2:0
[-P-] [ -] www-client/seamonkey-2.4.1-r1:0
[-P-] [ -] www-client/seamonkey-2.7.1:0
[-P-] [ ] www-client/seamonkey-2.7.1-r1:0
[-P-] [ ] www-client/seamonkey-2.8:0
[-P-] [ ] www-client/seamonkey-2.8-r1:0
Please bring this up on the gentoo-dev mailing list.
Reopening as requested in the discussion on the mailing list:
I cannot reassign so I am just adding relevant herds to the CC list.
based off a quick look appears that vpx is pulling in the lib but does not give control as to weather or not to disable/enable it. As far as mozilla packages, I am in no way looking to change the useflag, this would for a rebuild for anyone that use an uDN system/world, without giving any tactical advantage.
webm makes more sense in the case of firefox, but eg, in the case of ffmpeg, vpx or libvpx makes more sense since it has a native vp8 decoder (always enabled), a webm demuxer and muxer (also always enabled) and vpx only enables the libvpx based decoder and encoder.
note also that webm is the container, vp8 the video codec, so technically they are two different things.