I am not sure that I can explain it correctly, but the idea consists of several elements that work together
1. add XSL as resource type to the database
2. add an event "Use XSL" similar to "HTML Body attributes" to the form that can contain a formula that results to a name of a xsl resource
3. add an event "Use XSL" to each Field, embedded element, section, table with the same behaviour as above (I know that tables and sections currently have no "events")
4. basically the same for views and columns
At runtime, if one of these forms or views is opened and the formula results to a XSL resource name, the web engine does not perform the standard rendering into html but instead reads the DXL of the form/view and applies the XSL to it. The XSL of fields or columns is "included" at the intended place resulting in a big XSL transformation and domino has no responsibility that there is any output at all but this gives XSL savvy users the possibility to override the standard transformation process of Notes Layout to HTML (or different XML, XUL or ...). And I think it would ease to implement intregation of any AJAX framework or things like Flex, OpenLaszlo and similar.
Those XSL elements would need some guidelines of how to match layout and content (e.g. for fields), but I think that is something to cope with, e.g. putting both DXL junks in a single stream. Or alternativly work similar to a 2005 Lotusphere presentation as a 2-step XSLT. Step1: Form DXL + XSL = generated XSL, Step 2: document DXL + generated XSL = output to browser
Sounds (and probably is) complicated and it would certainly need much more discussion if considered as a feature.