Feel free to propose e.g. Accept-Media to httpbis[1]. Bandwidth
negotiation would be most useful.
Do make note of the dynamic nature of many viewports* and the fact that
user agents may wish to render resources to multiple medias. The latter
is rare enough to tolerate an extra roundtrip. Resizing viewports,
however, should ideally not require a roundtrip.
For fast resizing, UAs must obtain layouts for all possible sizes before
viewport resize. On fullscreen systems, these sizes might as few as one
or two. Thing is, user agents could lay self-contained elements out
recursively without author stylesheets.
User agents are all around much better suited to laying out
self-contained elements than authors are. User agents can dynamically
and intelligently apply new styles customized for their viewport. All
authors have to do is semantically use <nav>, <aside> and <link> (so UAs
can place them consistently) and group their content.
*Viewport: A window to which an user agent renders a document [2]
1: http://www.ietf.org/html.charters/httpbis-charter.html
2: http://www.w3.org/TR/CSS21/visuren.html#viewport
