So, let me get this straight.

First, sending XHTML as text/html is considered harmful.

Second, documents treated as XML are not loaded incrementally [1] in Firefox [2]. See Hixie's test cases for the effect. (Some version of IE seems to have the same feature, but Opera 9.0 does load the XML version incrementally [3].)

Thus: don't use XHTML?

[1] It's a pretty long-living bug that.

[2] Though I don't know whether Opera just ignores the mime type. Probably not.

[3] Henri Sivonen is working on making the XML Content Sink in Firefox incremental.