Just came across a small tip that might be useful for somebody.
If you want to merge / combine HTML from an external system into a Domino page then one option is to attach a file resource to the application and then in the form where you want the HTML rendered use the Create / Resource / Insert Resource option.
You end up with a small placeholder icon:
This technique means the file contents are added Server Side rather then using client side technologies like AJAX which is important for SEO purposes.
The file resource can be refreshed and the contents will be updated.
I see this being useful where you have a build system which generates HTML – like NodeJS and handlebars but you need to use an existing Domino application to host the content.
DXL can be used to update the file resource and if you used something like a Java agent you could pull in the HTML contents for the file resource via a URL.