Browser tracking across different sites is certainly a major privacy
concern and one that is more acute when the boundaries between sites and
browsers blur—or disappear altogether. That seems to be the underlying
tension in a “discussion” of an only tangentially related proposal being
made by Google to the W3C Technical
Architecture Group (TAG). The proposal would change the handling of
the User-Agent headers sent by browsers, but the discussion turned
to the unrelated X-Client-Data header that Chrome sends to
Google-owned sites. The connection is that in
both cases
some feel that the web-search giant is misusing its position to the detriment of
its users and its competitors in the web ecosystem.

Source: LWN.net – [$] Browsers, web sites, and user tracking