I have 2 HTML files, suppose
<ui:include src="b.xhtml" />
Surprisingly, the same question was asked and it is possible: HTML5 include file
Use the object tag:
<object name="foo" type="text/html" data="foo.inc"/>
foo.inc should include valid HTML.
I tested it on Konqueror, Firefox and Chromium.
If you find it useful (I do), please upvote Rafa answer (not mine) because "it is not possible" is spreading like disease.