There's no standard that defines what to put but Gecko uses the xhtml and mathml namespaces as requiredExtensions to indicate support. I believe Webkit may have followed suit but I've not tested that.
--- In firstname.lastname@example.org, "dark3251" <dark3251@...> wrote:
> I am trying to create an alternative display for user-agents that don't
> support <foreignObject>. There is a pretty good example in the SVG
> documentation here: Example
> The most important line is: <foreignObject width="100" height="50"
> However, under the "requiredExtensions" attribute of the <foreignObject>
> there is an example placeholder... What would I actually put here to
> denote that xhtml is the requiredExtension?
> [Non-text portions of this message have been removed]