Surprisingly, the same question was asked and it is possible: HTML5 include file
Rafa's answer:
Use the object tag:
foo.inc should include valid HTML.
I tested it on Konqueror, Firefox and Chromium. 
Note you must use a separate 
If you find it useful (I do), please upvote Rafa answer (not mine) because "it is not possible" is spreading like disease.