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 tag otherwise any content after it gets discarded.
If you find it useful (I do), please upvote Rafa answer (not mine) because "it is not possible" is spreading like disease.