Language API: how to embed languages within each other?

Keith Lea wrote:

I'd like to embed Java code into a custom language. I can't seem to find
how the HTML-JS-CSS integration works in the JS plugin source. Can
someone give me a pointer about where to look for this? Thanks.

I know this isn't really what you're asking for, but I filed
http://www.jetbrains.net/jira/browse/IDEABKL-3328 for the possibility to embed custom
languages into existing ones, especially any XML-dialect.

Oh, and to me it looks like the HTML-JS-CSS support is totally hard-wired into the IDEA
core. There's the JSHtmlBridge plugin that connects the JS plugin with the core, but
everything of interest (recognition of script-sections and event-handlers) is done on a
much deeper level. There doesn't seem to be any way to connect arbitrary languages to
sections of another.

Sascha