HTTP requests may include an "Accept" header, which indicates which MIME types the requesting user agent can understand, including markup types (e.g. XHTML), image and video formats, script, and so on. The Accept header tells you about a fundamental user agent capability: what kinds of markup the user agent understands. But this is all it reveals. For better or worse, more details device capability is often determined from the User-Agent header.
Note that the MIME types are listed in descending order of preference, and can even include an optional "q=" parameter specifying how desirable the content type is on a scale of 0.0 to 1.0 (see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html).
A typical header value might look like this:
Accept: application/vnd.wap.wmlc;Type=4365, application/vnd.wap.wmlc;Type=4360, application/vnd.wap.wmlc;Type=1108, application/vnd.wap.wmlc;Level=1.3, application/vnd.wap.wmlc, application/vnd.wap.wmlscriptc, application/vnd.wap.multipart.related, application/vnd.wap.multipart.mixed, application/x-up-device, application/vnd.phonecom.mmc-wbxml;Type=4364, application/vnd.phonecom.mmc-wbxml, application/vnd.phonecom.im, application/octet-stream, application/vnd.openwave.pp, application/vnd.wap.sic, application/vnd.wap.slc, application/vnd.wap.coc, application/vnd.uplanet.bearer-choice-wbxml, application/vnd.uplanet.signal, application/vnd.uplanet.cacheop-wbxml, application/vnd.uplanet.alert-wbxml, image/vnd.wap.wbmp;Type=0, application/*, audio/iMelody, audio/midi,text/x-hdml, image/mng,image/x-mng,video/mng,video/x-mng,image/bmp,text/vnd.wap.wml
It is not uncommon for user agents to specify */* as accepted content type. This means that it pretends to be able to accept any kind of content. This claim should be viewed with skepticism in the context of mobile devices, which are typically more limited in what formats and MIME types they support.
It is also not uncommon for WAP gateways (see for example this description: http://www.pcmag.com/encyclopedia_term/0,2542,t=WAP+gateway&i=54200,00.asp) to add additional MIME types to the Accept header's value. These may be added because the gateway can understand a format, for example image/png, which the request mobile user agent cannot, but which the gateway can transform into another format that is understood.
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